Time Glossary

ACDT (Australian Central Daylight Time) ACDT (Australian Central Daylight Time)

The daylight saving time observed in South Australia during Australian summer (UTC+10:30). Not observed in the Northern Territory.

ACST (Australian Central Standard Time) ACST (Australian Central Standard Time)

The standard time used in Australia's Northern Territory and South Australia (UTC+9:30). Australia's most unusual offset, sitting between UTC+9 and UTC+10.

AEDT (Australian Eastern Daylight Time) AEDT (Australian Eastern Daylight Time)

The daylight saving time observed in New South Wales, Victoria, Tasmania, and the ACT during Australian summer (UTC+11). Queensland does not observe AEDT.

AWST (Australian Western Standard Time) AWST (Australian Western Standard Time)

The standard time used in Western Australia (UTC+8). Western Australia does not observe daylight saving time, making AWST fixed year-round.

CEST (Central European Summer Time) CEST (Central European Summer Time)

The daylight saving time observed in Central European countries during summer (UTC+2). Countries include Germany, France, Italy, Spain, Poland, and others. Clocks spring forward 1 hour from CET.

EAT (East Africa Time) EAT (East Africa Time)

The time zone used in East African countries including Kenya, Ethiopia, Tanzania, and Uganda (UTC+3). No daylight saving is observed in this region.

EDT (Eastern Daylight Time) EDT (Eastern Daylight Time)

The daylight saving time observed on the U.S. and Canadian East Coast during summer (UTC-4). Major cities include New York, Boston, Toronto, and Miami.

EEST (Eastern European Summer Time) EEST (Eastern European Summer Time)

The daylight saving time observed in Eastern European countries such as Greece, Romania, Bulgaria, and Finland during summer (UTC+3). Corresponds to when clocks spring forward from EET.

NZDT (New Zealand Daylight Time) NZDT (New Zealand Daylight Time)

The daylight saving time observed in New Zealand during Southern Hemisphere summer (UTC+13). Clocks spring forward 1 hour from NZST.

PDT (Pacific Daylight Time) PDT (Pacific Daylight Time)

The daylight saving time observed on the U.S. and Canadian Pacific Coast during summer (UTC-7). States include California, Oregon, Washington, and British Columbia. Clocks spring forward from PST.

SAST (South Africa Standard Time) SAST (South Africa Standard Time)

The time zone used in South Africa, Lesotho, Swaziland, and Zimbabwe (UTC+2). South Africa does not observe daylight saving time.

WAT (West Africa Time) WAT (West Africa Time)

The time zone used in most of West and Central Africa, including Nigeria, Cameroon, and Angola (UTC+1). Corresponds to CET without daylight saving.

WEST (Western European Summer Time) WEST (Western European Summer Time)

The daylight saving time observed in Portugal, Ireland, and the Canary Islands during summer (UTC+1). Corresponds to when clocks spring forward from WET.

WITA (Central Indonesian Time) WITA (Central Indonesian Time)

The time zone covering Indonesia's central islands including Bali, Sulawesi, Lombok, and Kalimantan (UTC+8). Indonesia is divided into three time zones.

WIT (Eastern Indonesian Time) WIT (Eastern Indonesian Time)

The time zone covering Eastern Indonesia including Papua, Maluku, and West Papua (UTC+9). The easternmost of Indonesia's three official time zones.

Aphelion Aphelion

The point in Earth's elliptical orbit farthest from the Sun, occurring around July 4–6 each year. At aphelion, Earth is approximately 152.1 million km from the Sun.

Astronomical Twilight Astronomical Twilight

The period when the Sun is between 12° and 18° below the horizon. During astronomical twilight, the sky is dark enough for most astronomical observations except for the faintest objects.

Axial Precession Axial Precession

The slow wobble of Earth's rotational axis, completing one full cycle approximately every 25,772 years (the Platonic Year). Precession causes the vernal equinox to drift through the zodiac constellations over millennia.

Axial Tilt (Obliquity) Axial Tilt (Obliquity)

The angle between Earth's rotational axis and the perpendicular to its orbital plane, currently approximately 23.44°. Axial tilt is responsible for the seasons and the variation in day length throughout the year.

Crepuscular Rays Crepuscular Rays

Shafts of sunlight separated by shadows, visible during twilight when the Sun is near the horizon. Although the rays appear to diverge from the Sun, they are actually parallel, an effect of perspective.

Declination (Solar) Declination (Solar)

The angular distance of the Sun north or south of the celestial equator, varying between approximately ±23.5° over a year due to Earth's axial tilt. Declination determines the length of day and the height of the Sun in the sky.

Hour Angle Hour Angle

The angle between the meridian and the hour circle of a celestial object, measuring how far the object has moved from its highest point (transit). Zero hour angle means the object is on the meridian at solar noon.

Midnight Sun Midnight Sun

The natural phenomenon where the Sun remains visible above the horizon at midnight, occurring in regions north of the Arctic Circle or south of the Antarctic Circle during their respective summers.

Nautical Twilight Nautical Twilight

The period when the Sun is between 6° and 12° below the horizon. During nautical twilight, the sea horizon is still visible, making celestial navigation with a sextant possible. Darker than civil twilight.

Nutation Nutation

A small, irregular oscillation of Earth's rotational axis superimposed on the larger precession cycle. The principal nutation period is approximately 18.6 years, caused mainly by the Moon's gravitational pull.

Perihelion Perihelion

The point in Earth's elliptical orbit closest to the Sun, occurring around January 3–4 each year. At perihelion, Earth is approximately 147.1 million km from the Sun — about 3% closer than at aphelion.

Polar Night Polar Night

The period during winter at high latitudes when the Sun does not rise above the horizon for 24 hours or more. The opposite of the midnight sun, it can last from days to months depending on latitude.

Right Ascension Right Ascension

The celestial equivalent of longitude, measured eastward along the celestial equator from the vernal equinox. Right ascension is expressed in hours, minutes, and seconds (0–24 h), and is used to locate celestial objects.

Solar Constant Solar Constant

The average energy flux received from the Sun per unit area at Earth's mean distance, approximately 1,361 W/m². It is not truly constant but varies slightly with the solar cycle.

Summer Solstice Summer Solstice

The day of the year with the longest daylight period, occurring when Earth's axial tilt is most inclined toward the Sun. In the Northern Hemisphere, it falls around June 20–21; in the Southern Hemisphere, around December 21–22.

TDB (Barycentric Dynamical Time) TDB (Barycentric Dynamical Time)

A relativistic time scale used in planetary ephemerides, measured at the barycenter of the Solar System. TDB accounts for relativistic effects due to Earth's varying distance from the Sun and differs from TT by less than 2 milliseconds.

Winter Solstice Winter Solstice

The day of the year with the shortest daylight period, occurring when Earth's axial tilt is most inclined away from the Sun. In the Northern Hemisphere, it falls around December 21–22; in the Southern Hemisphere, around June 20–21.

Academic Year Academic Year

The annual period of instruction at schools and universities. In Korea it runs March to February; in the US and UK, September to June/July; in Japan, April to March.

Astronomical Year Numbering Astronomical Year Numbering

A year numbering system used by astronomers that includes a year zero (0 AD) and uses negative numbers for BC years. In this system, 1 BC = year 0, and 2 BC = year -1.

Aztec Calendar Aztec Calendar

The calendar system used by the Aztec civilization, consisting of two interlocking cycles: the 365-day xiuhpōhualli (solar year) and the 260-day tōnalpōhualli (ritual calendar). Together they form a 52-year Calendar Round.

Bengali Calendar (Bangla San) Bengali Calendar (Bangla San)

The traditional solar calendar used in Bangladesh and the West Bengal region of India. The Bengali New Year (Pohela Boishakh) falls around April 14–15 and is a major cultural celebration.

Bikram Sambat Bikram Sambat

The official calendar of Nepal, a lunisolar calendar approximately 56.7 years ahead of the Gregorian calendar. Nepal celebrates its New Year (Nava Barsha) in mid-April.

Chinese Calendar (農曆) Chinese Calendar (農曆)

The traditional lunisolar calendar of China, used to determine holidays and festivals such as Chinese New Year. The calendar follows lunar months while keeping alignment with the solar year through leap months.

Chinese/Korean Zodiac Chinese/Korean Zodiac

A 12-year cycle of animals (Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog, Pig) used in East Asian culture.

Coptic Calendar Coptic Calendar

The liturgical calendar of the Coptic Orthodox Church, derived from the ancient Egyptian calendar with 13 months. It is 13 years behind the Gregorian calendar, and still used officially in Ethiopia as the basis of the Ethiopian calendar.

Discordian Calendar Discordian Calendar

A satirical calendar from the Principia Discordia (1963) with 5 seasons of 73 days each and year 1 set at 1166 BCE. While humorous, it is occasionally referenced in computing and hacker culture.

Ethiopian Calendar Ethiopian Calendar

The calendar used in Ethiopia, based on the Coptic calendar with 13 months (12 months of 30 days plus a 13th month of 5–6 days). Ethiopia is approximately 7–8 years behind the Gregorian calendar.

Fiscal Year Fiscal Year

A 12-month period used for financial reporting that may differ from the calendar year. In Korea and the US it aligns with January-December; Japan uses April-March; the UK uses April-March.

French Republican Calendar French Republican Calendar

A calendar system introduced during the French Revolution (1793–1806), with 12 months of 30 days each and 5–6 extra days. It abolished Sunday and redesigned the week into 10-day décades.

Gapja (Sexagenary Cycle) Gapja (Sexagenary Cycle)

A 60-year cycle combining 10 Heavenly Stems and 12 Earthly Branches, used in Korean and East Asian calendrical systems for year naming.

Gregorian Calendar Gregorian Calendar

The solar calendar introduced by Pope Gregory XIII in 1582, now the international civil standard. Corrected the Julian calendar's drift by refining the leap year rule.

Hebrew Calendar Hebrew Calendar

A lunisolar calendar used in Judaism with months based on lunar cycles and leap months added to synchronize with the solar year. Currently in its 58th century.

Hindu Calendar (Panchang) Hindu Calendar (Panchang)

A family of lunisolar calendars used in Hinduism, with regional variations across India. The Panchang specifies five elements: day, lunar day, zodiac sign, auspicious moment, and half-day.

Holocene Calendar Holocene Calendar

A proposed calendar system that adds 10,000 years to the current Gregorian year to include the entire span of human civilization. Year 2025 CE becomes 12025 HE (Human Era). Popularized by the Kurzgesagt YouTube channel.

Islamic Calendar (Hijri) Islamic Calendar (Hijri)

A purely lunar calendar with 12 months of 29 or 30 days, totaling about 354 days per year. Used to determine Islamic holidays like Ramadan. Year count begins from the Hijra (622 CE).

ISO Week ISO Week

A week numbering system defined by ISO 8601 where weeks start on Monday and the first week of the year contains the first Thursday. A year has 52 or 53 ISO weeks.

Japanese Era Calendar (元号) Japanese Era Calendar (元号)

Japan's traditional calendar system where years are counted within imperial eras (gengō). The current era is Reiwa (令和), begun in 2019 with Emperor Naruhito's accession. Widely used in official documents.

Juche Calendar Juche Calendar

The calendar used in North Korea, counting years from the birth of Kim Il-sung in 1912 (Juche 1). Year 2025 CE corresponds to Juche 114. Months and days follow the Gregorian system.

Julian Calendar Julian Calendar

A solar calendar introduced by Julius Caesar in 46 BC with a leap year every 4 years. Accumulated about 10 days of error by the 16th century, leading to its replacement by the Gregorian calendar.

Leap Year Leap Year

A year with 366 days instead of 365, adding February 29. Occurs every 4 years, except for years divisible by 100 but not by 400 (e.g., 2000 was a leap year, 1900 was not).

Lunar Calendar Lunar Calendar

A calendar based on the phases of the Moon. The Korean lunar calendar (음력) is used alongside the solar calendar for traditional holidays like Seollal and Chuseok.

Maya Long Count Calendar Maya Long Count Calendar

A non-repeating calendar used by the ancient Maya civilization to record historical and mythological dates. It counts days from a mythological creation date, famously 'resetting' on December 21, 2012.

Minguo Calendar (Republic of China) Minguo Calendar (Republic of China)

Taiwan's official calendar, counting years from the founding of the Republic of China in 1912. Year 2025 CE corresponds to Minguo 114. Used alongside Gregorian dates in Taiwan.

Nowruz (New Year) Nowruz (New Year)

The Persian and Iranian New Year, celebrated at the exact moment of the spring equinox (around March 20–21). Nowruz is a UNESCO Intangible Cultural Heritage observed by over 300 million people across the Middle East and Central Asia.

Persian Calendar (Solar Hijri) Persian Calendar (Solar Hijri)

The official calendar of Iran and Afghanistan, based on the solar year with 12 months and starting from the Islamic Hijra. The most accurate solar calendar in widespread use, with years beginning at the spring equinox.

Proleptic Gregorian Calendar Proleptic Gregorian Calendar

An extension of the Gregorian calendar to dates before its official introduction in 1582. Used in historical and astronomical calculations to avoid gaps. ISO 8601 uses the proleptic Gregorian calendar.

Quarter (Calendar Quarter) Quarter (Calendar Quarter)

A period of three consecutive months that divides a year into four equal parts (Q1–Q4). Widely used in business, finance, and government reporting. Some companies use non-calendar fiscal quarters.

Solar Hijri Calendar Solar Hijri Calendar

A solar calendar used in Iran and Afghanistan that begins at the vernal equinox. The months are named after the 12 constellations of the zodiac. It is the most accurate solar calendar in current official use.

Solar Terms (24 Jeolgi) Solar Terms (24 Jeolgi)

A system of 24 periods in traditional East Asian calendars based on the Sun's position. Used to guide agricultural activities (e.g., 입춘, 하지, 추분, 동지).

Thai Buddhist Calendar Thai Buddhist Calendar

The official calendar of Thailand, which adds 543 years to the Gregorian year (BE = Buddhist Era). For example, 2025 CE corresponds to 2568 BE. Used alongside the Gregorian calendar in official documents.

Analemma Analemma

A figure-eight curve traced by the Sun's position in the sky when observed at the same time each day over a year. It results from Earth's axial tilt and orbital eccentricity.

Apparent Solar Time Apparent Solar Time

The actual time as indicated by a sundial, based on the Sun's real position in the sky. Varies from mean solar time by up to 16 minutes due to Earth's elliptical orbit and axial tilt.

Atomic Clock Atomic Clock

A clock that uses the resonance frequency of atoms (typically cesium-133) to keep time with extreme precision. Modern cesium clocks lose less than 1 second in 100 million years.

Biological Clock Biological Clock

The internal molecular mechanism in living organisms that generates rhythmic oscillations roughly matching the 24-hour day. In mammals, the master clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus.

Business Hours Business Hours

The standard hours during which a business operates and is open to customers, typically 9 AM to 5 PM on weekdays in Western countries. Business hours vary globally and affect cross-timezone scheduling.

Chronemics Chronemics

The study of how time is used and perceived as a form of nonverbal communication in different cultures. It examines concepts like punctuality, waiting time, and the pace of interaction.

Chronobiology Chronobiology

The scientific study of cyclic biological phenomena and their adaptation to solar and lunar rhythms. It encompasses circadian, ultradian, and infradian biological cycles.

Chronological Order Chronological Order

The arrangement of events in the sequence in which they occurred in time, from earliest to latest. Chronological order is the default expectation in history, journalism, and database queries using ORDER BY time.

Chronotherapy Chronotherapy

A treatment method that schedules medical interventions, such as drug administration or light therapy, at specific times aligned with the patient's biological clock. Used for conditions like depression, cancer, and circadian rhythm disorders.

Chronotype Chronotype

An individual's natural preference for sleeping and waking at specific times, commonly described as 'morning types' (larks) or 'evening types' (owls). Chronotype is influenced by genetics and age.

Circadian Rhythm Circadian Rhythm

The roughly 24-hour internal biological cycle that regulates sleep, wakefulness, and many physiological processes. Circadian rhythms are driven by light exposure and reset daily by environmental cues.

Clock Face Clock Face

The dial or display of a clock showing hours, minutes, and sometimes seconds, typically as a numbered circle (analog) or digital readout. The 12-hour analog clock face is one of humanity's most universally recognized interfaces.

Countdown Timer Countdown Timer

A device or software that counts down from a set time to zero, used for events like rocket launches, New Year celebrations, sports timing, and cooking. A foundational interface for time-based applications.

Daylight Saving Debate Daylight Saving Debate

The ongoing societal and political discussion about whether daylight saving time should be kept, abolished, or made permanent. Health experts, economists, and public opinion are divided, with most sleep researchers advocating for abolition.

Daylight Saving Time History Daylight Saving Time History

First proposed by Benjamin Franklin in 1784 and implemented widely during World War I to conserve fuel. Many countries have since abolished it; Korea used it briefly in 1948-1951 and 1955-1960.

Deadline Timezone Ambiguity Deadline Timezone Ambiguity

The confusion that arises when deadlines are not specified with an explicit time zone, leading to misinterpretations (e.g., 'midnight EST vs. midnight local time'). Best practice is to always specify the timezone for any deadline.

Deep Time Deep Time

The concept of geologic time spanning millions to billions of years, far beyond human intuitive experience. Coined by John McPhee, it is essential for understanding Earth's history, evolution, and cosmology.

DST (Daylight Saving Time) DST (Daylight Saving Time)

The practice of advancing clocks by one hour during warmer months to extend evening daylight. About 70 countries observe DST.

Dual Time Zone Display Dual Time Zone Display

A watch or device feature that simultaneously displays the time in two different time zones. Common among frequent travelers and remote teams working across multiple time zones.

Entrainment (Circadian) Entrainment (Circadian)

The process by which the body's internal clock synchronizes with external zeitgebers such as light. Without entrainment, the circadian clock would drift away from the 24-hour day.

Equation of Time Equation of Time

The difference between apparent solar time and mean solar time, caused by Earth's elliptical orbit and axial tilt. Ranges from about -14 to +16 minutes throughout the year.

Half-Hour Offset Half-Hour Offset

A time zone offset that is a non-integer multiple of one hour, such as UTC+5:30 (India), UTC+5:45 (Nepal), or UTC+9:30 (Australia's Northern Territory). These offsets arise from historical political and geographic reasons.

Happy Hour Happy Hour

A period at a bar or restaurant, typically in the late afternoon or early evening, when drinks or food are offered at discounted prices. The concept originated in the U.S. Navy in the 1920s as recreational time for sailors.

International Date Line International Date Line

An imaginary line roughly following 180 degrees longitude where the date changes. Crossing westward adds a day; crossing eastward subtracts a day.

Jet Lag Jet Lag

A temporary sleep disorder caused by rapid travel across multiple time zones, disrupting the body's circadian rhythm. Symptoms include fatigue, insomnia, and difficulty concentrating.

Leap Second Leap Second

A one-second adjustment applied to UTC to keep it close to mean solar time. Positive or negative, decided by the International Earth Rotation Service.

Local Mean Time (LMT) Local Mean Time (LMT)

The mean solar time at a specific longitude, before standardized time zones were adopted. Every degree of longitude corresponds to a 4-minute difference in LMT.

Magic Hour Magic Hour

Another term for golden hour in photography — the brief period after sunrise or before sunset when sunlight is soft, warm, and directional, producing flattering lighting for portraits and landscapes.

Mean Solar Time Mean Solar Time

An averaged form of solar time that smooths out the variations in the Sun's apparent motion throughout the year. The basis for civil timekeeping before atomic clocks.

Military Time (24-Hour Clock) Military Time (24-Hour Clock)

A timekeeping system using 0-23 hours instead of AM/PM. Used by military, aviation, healthcare, and most non-English-speaking countries.

Monochronic Culture Monochronic Culture

A cultural orientation where tasks are handled one at a time and schedules are strictly followed. Northern European and North American business cultures are often described as monochronic.

Permanent DST Permanent DST

A proposed or enacted policy to remain on daylight saving time year-round, eliminating the biannual clock change. Proponents cite economic and health benefits; critics argue it creates dark winter mornings.

Phase Shift Phase Shift

A temporary advance or delay in an organism's circadian rhythm caused by a zeitgeber at an unusual time. Jet lag and shift work both cause phase shifts that require days to resolve.

Polychronic Culture Polychronic Culture

A cultural orientation where multiple tasks are handled simultaneously and schedules are flexible. Latin American, Middle Eastern, and many Asian cultures are often described as polychronic.

Pomodoro Technique Pomodoro Technique

A time management method where work is divided into 25-minute focused sessions (pomodori) separated by 5-minute breaks. Developed by Francesco Cirillo in the late 1980s using a tomato-shaped timer.

Prime Meridian Prime Meridian

The meridian at 0 degrees longitude, passing through Greenwich, England. The starting point for measuring longitude east and west.

Prime Time Prime Time

The block of television programming aired during the evening hours when viewership is at its highest, typically 8–11 PM in the U.S. Advertising rates are highest during prime time due to the large audience.

Quartz Clock Quartz Clock

A clock that uses the vibration of a quartz crystal (32,768 Hz) as its timekeeping element. Invented in 1927, quartz clocks are accurate to within a few seconds per month.

Rush Hour Rush Hour

The period of day when traffic congestion on roads or public transit is at its peak, typically in the morning (7–9 AM) and evening (5–7 PM) on weekdays. Rush hours vary by city and local work culture.

Season Season

One of the four annual divisions of the year (spring, summer, autumn, winter) resulting from Earth's axial tilt as it orbits the Sun. Seasons are opposite in the Northern and Southern Hemispheres.

Sidereal Time Sidereal Time

Time measured by the apparent motion of stars rather than the Sun. A sidereal day is about 23 hours 56 minutes, roughly 4 minutes shorter than a solar day.

Siesta Siesta

A traditional afternoon rest or nap observed in hot climates, especially in Mediterranean countries, Latin America, and parts of Asia. Typically occurs between 1–4 PM and is closely tied to local solar time.

Sleep Debt Sleep Debt

The cumulative effect of not getting enough sleep over time. Sleep debt impairs cognitive function, mood, and physical health, and is partially recoverable through extended sleep.

Social Jet Lag Social Jet Lag

The chronic discrepancy between an individual's biological clock and their socially imposed schedule, such as early work or school start times. Common in evening chronotypes forced into early schedules.

Solar Time Solar Time

Time determined by the position of the Sun. Apparent solar time varies throughout the year; mean solar time averages these variations.

Standard Time Standard Time

The uniform time used in a region during winter months (or year-round in regions without DST). Standard time is based on a fixed UTC offset and corresponds to the official time zone of a region.

Standard Time Debate Standard Time Debate

The ongoing policy discussion over whether to permanently adopt standard time or daylight saving time, or to abolish DST altogether. Health researchers generally favor permanent standard time for its alignment with the solar cycle.

Temporal Resolution Temporal Resolution

The precision with which a system can distinguish between events occurring at different points in time. High temporal resolution is critical in scientific instruments, computing, and financial trading.

Time Affluence Time Affluence

The feeling of having sufficient discretionary time to pursue leisure, relationships, and personal goals. Research shows time affluence contributes more to subjective well-being than material wealth beyond a certain income level.

Time Blindness Time Blindness

A difficulty in accurately perceiving the passage of time, often associated with ADHD and other neurological conditions. Individuals with time blindness may struggle with punctuality and time management.

Time Blocking Time Blocking

A productivity method where the day is divided into blocks of time, each dedicated to a specific task or category of tasks. Popularized by productivity experts as a way to minimize context switching.

Time Boxing Time Boxing

A time management technique where a fixed maximum amount of time is allocated for an activity. Widely used in agile software development to scope sprints and prevent scope creep.

Time Capsule Time Capsule

A container of artifacts or information intentionally stored to be opened at a future date, preserving a snapshot of the current time for future observers. Time capsules are used in architecture, science, and cultural memory projects.

Time Dilation Time Dilation

A phenomenon predicted by Einstein's theory of relativity where time passes more slowly for objects moving at high speeds or in stronger gravitational fields. GPS satellites must account for this effect.

Time Perception Time Perception

The subjective experience of the passage of time, which varies with emotional state, attention, and age. Time feels faster during engaging activities and slower during boredom or stress.

Time Poverty Time Poverty

The state of having insufficient discretionary time after fulfilling work and care obligations. Research links time poverty to reduced well-being and is more prevalent among working parents.

Timestamp Timestamp

A recorded time marking when an event occurred, typically encoded as a date-time string or numeric value. Timestamps are fundamental to logging, version control, databases, and digital forensics.

Time Synchronization Time Synchronization

The process of coordinating clocks in a distributed system to agree on a common time. Essential for ensuring correct transaction ordering, log correlation, and security certificate validation across networked devices.

Time Zone Abbreviation Time Zone Abbreviation

Short codes representing time zones (e.g., KST, EST, GMT). Abbreviations can be ambiguous — 'CST' can mean Central Standard Time (US), China Standard Time, or Cuba Standard Time.

Time Zone Conversion Time Zone Conversion

The process of converting a date and time value from one time zone to another. Correct conversion requires knowledge of UTC offsets, DST rules, and historical timezone changes stored in the tz database.

Time Zone Politics Time Zone Politics

The influence of political, economic, and national identity factors on a country's choice of time zone or DST policy. Examples include China's single time zone, Spain's alignment with CET, and Russia's occasional reforms.

UTC-12 (Baker Island Time) UTC-12 (Baker Island Time)

The most behind time zone on Earth, corresponding to Baker Island and Howland Island, both uninhabited U.S. territories. No permanent human population uses UTC-12.

UTC+14 (Line Islands Time) UTC+14 (Line Islands Time)

The most advanced time zone on Earth, used by Kiribati's Line Islands. UTC+14 is ahead of UTC-12 by 26 hours, meaning parts of Earth experience different calendar dates simultaneously.

UTC Offset UTC Offset

The difference between a timezone and UTC, expressed in hours and minutes (e.g., UTC+9 for Korea, UTC-5 for US Eastern).

Witching Hour Witching Hour

Traditionally, midnight — the time when supernatural entities were believed to be most active in folklore. In modern usage it can also refer to 3 AM, and colloquially to any late-night period of peak activity.

World Clock World Clock

A clock or display showing the current time in multiple cities or time zones simultaneously. Widely used in international business, broadcasting studios, and airports to coordinate across geographic locations.

Zeitgeber Zeitgeber

External environmental cues (from German: 'time giver') that synchronize an organism's biological clock to the 24-hour cycle. Light is the most powerful zeitgeber; others include meals, temperature, and social interaction.

GMT (Greenwich Mean Time) GMT (Greenwich Mean Time)

The mean solar time at the Royal Observatory in Greenwich, London. Historically the global time standard, now largely replaced by UTC for technical purposes.

TAI (International Atomic Time) TAI (International Atomic Time)

A high-precision atomic coordinate time standard based on 450+ atomic clocks worldwide. UTC is derived from TAI with leap second adjustments.

UTC (Coordinated Universal Time) UTC (Coordinated Universal Time)

The primary time standard by which the world regulates clocks and time. UTC is the successor to GMT and is used as the basis for civil time worldwide.

Azimuth Azimuth

The horizontal angle of the Sun measured clockwise from true north (0-360 degrees). At sunrise, the azimuth varies seasonally — near 90 degrees at equinoxes and farther from it at solstices.

Blue Hour Blue Hour

The twilight period before sunrise or after sunset when the sky takes on a deep blue tone. Occurs when the Sun is between 4 and 8 degrees below the horizon.

Civil Twilight Civil Twilight

The period when the Sun is between 0 and 6 degrees below the horizon. There is enough natural light for most outdoor activities without artificial lighting.

Equinox Equinox

When day and night are approximately equal in length. Vernal equinox (~March 20) marks spring; autumnal equinox (~September 23) marks autumn.

Golden Hour Golden Hour

The period shortly after sunrise or before sunset when sunlight is warm, soft, and golden-hued. Roughly when the Sun is between 0 and 10 degrees above the horizon, prized by photographers.

Nadir Nadir

The point on the celestial sphere directly below an observer, diametrically opposite to the zenith. Represents the lowest point of the Sun's path at solar midnight.

Solar Elevation (Altitude) Solar Elevation (Altitude)

The angle of the Sun above the horizon, ranging from -90 to +90 degrees. Zero at sunrise/sunset, maximum at solar noon. Determines shadow length and solar energy intensity.

Solar Noon Solar Noon

The moment when the Sun crosses the local meridian and reaches its highest point in the sky for the day. Does not necessarily coincide with 12:00 clock time due to time zones and DST.

Solstice Solstice

When the Sun reaches its highest or lowest point in the sky at noon. Summer solstice (~June 21) is the longest day; winter solstice (~December 22) is the shortest.

Sunrise Sunrise

The moment the upper edge of the Sun appears on the horizon in the morning. Exact time varies by latitude, date, and atmospheric conditions.

Sunset Sunset

The moment the upper edge of the Sun disappears below the horizon in the evening. Twilight begins after sunset.

Twilight Twilight

The period between sunrise/sunset and full darkness. Civil twilight ends when the Sun is 6 degrees below the horizon.

Zenith Zenith

The point in the sky directly above an observer. The Sun passes through the zenith at solar noon only in the tropics (between latitudes 23.5N and 23.5S).

Chrony Chrony

A versatile implementation of the NTP protocol for Linux systems, designed to perform well even with intermittent network connections. Chrony is the default NTP client on Red Hat and Ubuntu 20.04+ systems.

Clock Drift Clock Drift

The gradual deviation of a computer clock from the true time due to imperfect oscillator frequency. All hardware clocks drift; NTP corrects this by periodically synchronizing with reference time servers.

Clock Skew Clock Skew

The difference in time between two clocks in a distributed system at a given moment. Clock skew can cause transaction ordering errors in databases and distributed applications; it is measured and bounded by protocols like NTP and PTP.

Cron Cron

A Unix/Linux job scheduler that runs commands at specified times using a five-field expression (minute, hour, day, month, weekday). For example, '0 9 * * 1' runs every Monday at 9 AM.

datetime (Programming Object) datetime (Programming Object)

A programming data type combining date and time information in a single object. Implementations exist in most languages (Python's datetime, Java's LocalDateTime, JavaScript's Date). Timezone-aware vs. naive datetimes is a critical distinction.

Epoch Epoch

A reference point in time from which a calendar or time system begins counting. The Unix epoch is January 1, 1970, 00:00:00 UTC.

GPS Time GPS Time

The time standard maintained by the GPS satellite constellation, continuous since January 6, 1980, and not adjusted for leap seconds. GPS time is currently ahead of UTC by 18 seconds (as of 2025).

IANA Time Zone IANA Time Zone

The standard timezone identifiers maintained by IANA (e.g., 'Asia/Seoul', 'America/New_York'). Used by most operating systems and programming languages.

ISO 8601 ISO 8601

An international standard for representing dates and times (e.g., '2026-02-24T15:30:00+09:00'). The format most widely used in APIs and data exchange.

Leap Smearing Leap Smearing

A technique used by Google, Amazon, and other cloud providers to gradually spread a leap second adjustment over hours instead of inserting it all at once, avoiding software glitches.

Monotonic Clock Monotonic Clock

A clock that only moves forward and is never adjusted backward, unlike a wall clock that can be set back by NTP. Used for measuring elapsed time intervals precisely in programming (e.g., Python's time.monotonic()).

Naive Datetime Naive Datetime

A datetime object that does not include timezone information. Naive datetimes are ambiguous and error-prone in applications serving multiple timezones. Best practice is to always use timezone-aware datetimes.

ntpd (NTP Daemon) ntpd (NTP Daemon)

The reference NTP implementation from the NTP Project, historically the most widely deployed time synchronization daemon. Largely replaced by Chrony on modern Linux distributions.

NTP (Network Time Protocol) NTP (Network Time Protocol)

A protocol for synchronizing computer clocks over a network. NTP can achieve accuracy within milliseconds on the internet.

NTP Stratum NTP Stratum

A hierarchical level in the NTP time distribution system. Stratum 0 devices are reference clocks (GPS, atomic). Stratum 1 servers connect directly to Stratum 0. Each layer adds latency and decreases accuracy.

Olson Database Olson Database

The common name for the IANA tz database, named after its original creator Arthur David Olson. It contains timezone identifiers like 'America/New_York' and historical DST rules for every recognized timezone.

POSIX Time POSIX Time

Effectively the same as Unix time — seconds elapsed since 1970-01-01T00:00:00Z. POSIX defines each day as exactly 86,400 seconds, deliberately ignoring leap seconds.

PTP (Precision Time Protocol) PTP (Precision Time Protocol)

IEEE 1588 standard for clock synchronization with sub-microsecond accuracy over local networks. Used in financial trading, telecommunications, and industrial automation where NTP is not precise enough.

RFC 3339 RFC 3339

An internet standard for date and time representation, based on ISO 8601 but stricter. Requires the 'T' separator and timezone offset (e.g., '2026-02-25T10:30:00+09:00'). Widely used in REST APIs.

SNTP (Simple Network Time Protocol) SNTP (Simple Network Time Protocol)

A simplified version of NTP for devices that require time synchronization but not full NTP accuracy. Suitable for IoT devices, embedded systems, and Windows desktop clients. Uses a single round-trip exchange.

strftime strftime

A C standard library function for formatting date/time values into strings using format codes (e.g., %Y for year, %m for month). Used in Python, PHP, Ruby, and many other languages.

systemd-timesyncd systemd-timesyncd

A lightweight SNTP-based time synchronization service built into systemd, suitable for most desktop and server systems. It cannot act as an NTP server but is simpler than ntpd or Chrony for client use.

TAI (International Atomic Time) (redirect) TAI (International Atomic Time) (redirect)

See 'atomic-time'. TAI is a continuous time scale maintained by over 400 atomic clocks worldwide, serving as the basis for UTC.

TAI-UTC Difference TAI-UTC Difference

The accumulated difference between TAI (International Atomic Time) and UTC due to inserted leap seconds. As of 2017, TAI is 37 seconds ahead of UTC.

Time Server Time Server

A network server that reads the actual time from a reference time source and distributes it to clients using NTP or similar protocols. Examples include pool.ntp.org and time.google.com.

time_t (C Time Type) time_t (C Time Type)

The C programming language data type used to represent time, typically as seconds since the Unix epoch. Historically 32-bit signed, which causes the Year 2038 Problem; modern systems use 64-bit time_t.

Timezone-Aware Datetime Timezone-Aware Datetime

A datetime object that includes explicit timezone information (e.g., UTC offset or IANA zone name). Timezone-aware datetimes eliminate ambiguity and are required for correct cross-timezone date arithmetic.

tz Database (tzdata) tz Database (tzdata)

The IANA-maintained database of the world's time zone rules and historical changes. Contains entries like 'Asia/Seoul' and 'America/New_York'. Updated several times per year as countries change their rules.

Unix Timestamp Unix Timestamp

The number of seconds elapsed since January 1, 1970, 00:00:00 UTC (the Unix epoch). Widely used in computing for representing time.

Wall Clock Time Wall Clock Time

The actual elapsed time measured by a clock on the wall, as opposed to CPU time or monotonic time. Wall clock time can jump forward or backward due to NTP adjustments, DST changes, or manual corrections.

Windows Time Service (W32tm) Windows Time Service (W32tm)

The built-in time synchronization service on Windows, using a simplified SNTP implementation. While functional for basic desktop use, it is less accurate than Chrony or ntpd for servers requiring precise time.

Year 2038 Problem (Y2K38) Year 2038 Problem (Y2K38)

A potential computing bug where 32-bit signed integer Unix timestamps will overflow on January 19, 2038, 03:14:07 UTC. Systems using 32-bit time_t will wrap to a negative value, interpreted as 1901.

AEST (Australian Eastern Standard Time) AEST (Australian Eastern Standard Time)

UTC+10, used by eastern Australia including Sydney and Melbourne. During DST (October-April), AEDT (UTC+11) is used in most states except Queensland.

AST (Arabia Standard Time) AST (Arabia Standard Time)

UTC+3, used by Saudi Arabia, Iraq, Kuwait, and other Persian Gulf countries. Same offset as Moscow Standard Time. No DST is observed.

BRT (Brasilia Time) BRT (Brasilia Time)

UTC-3, the official time zone of Brazil's capital Brasilia and most of eastern Brazil. Brazil abolished DST in 2019.

CET (Central European Time) CET (Central European Time)

UTC+1, used by most of continental Europe during standard time. During DST, CEST (UTC+2) is used.

CST (Central Standard Time) CST (Central Standard Time)

UTC-6, used by the central United States during standard time. During DST, Central Daylight Time (CDT, UTC-5) is used instead. Note: CST is ambiguous and can also refer to China Standard Time (UTC+8).

EET (Eastern European Time) EET (Eastern European Time)

UTC+2, used by eastern European and Middle Eastern countries including Finland, Greece, Egypt, and Israel. During DST, EEST (UTC+3) is used.

EST (Eastern Standard Time) EST (Eastern Standard Time)

UTC-5, used by eastern North America during standard time. During daylight saving time, Eastern Daylight Time (EDT, UTC-4) is used instead.

HKT (Hong Kong Time) HKT (Hong Kong Time)

UTC+8, the time zone used in Hong Kong. Same offset as China Standard Time (CST) and Philippine Time (PHT). Hong Kong does not observe DST.

ICT (Indochina Time) ICT (Indochina Time)

UTC+7, used by Thailand, Vietnam, Laos, and Cambodia. None of these countries observe DST.

IST (India Standard Time) IST (India Standard Time)

UTC+5:30, the single time zone used across all of India. One of the few major time zones with a 30-minute offset. India does not observe DST.

JST (Japan Standard Time) JST (Japan Standard Time)

The time zone used in Japan (UTC+9). Same offset as KST. Japan does not observe daylight saving time.

KST (Korea Standard Time) KST (Korea Standard Time)

The time zone used in South Korea (UTC+9). Korea does not observe daylight saving time, so KST remains the same year-round.

MSK (Moscow Standard Time) MSK (Moscow Standard Time)

UTC+3, the time zone of Moscow and most of European Russia. Russia spans 11 time zones from UTC+2 to UTC+12 and does not observe DST since 2014.

MST (Mountain Standard Time) MST (Mountain Standard Time)

UTC-7, used by the mountain region of North America during standard time. Arizona does not observe DST and stays on MST year-round.

NZST (New Zealand Standard Time) NZST (New Zealand Standard Time)

UTC+12, the standard time zone for New Zealand. During DST (September-April), NZDT (UTC+13) is used, making it one of the first places to see a new day.

PHT (Philippine Time) PHT (Philippine Time)

UTC+8, the standard time zone of the Philippines. Same offset as China, Hong Kong, and Singapore. The Philippines does not observe DST.

PST (Pacific Standard Time) PST (Pacific Standard Time)

UTC-8, used by western North America during standard time. During DST, Pacific Daylight Time (PDT, UTC-7) is used instead.

SGT (Singapore Time) SGT (Singapore Time)

UTC+8, the time zone used in Singapore. Despite being geographically closer to UTC+7, Singapore uses UTC+8 to align with major trading partners like Hong Kong and China.

WET (Western European Time) WET (Western European Time)

UTC+0, used by Portugal, the UK, Ireland, and Iceland during standard time. Equivalent to GMT. During DST, WEST (UTC+1) is used except in Iceland.

WIB (Western Indonesian Time) WIB (Western Indonesian Time)

UTC+7, used in western Indonesia including Jakarta and Sumatra. Indonesia spans three time zones: WIB (UTC+7), WITA (UTC+8), and WIT (UTC+9).