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Document JapaneseCalendar.MinSupportedDateTime breaking change in .NET 11 Preview 1#50920

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copilot/update-japanese-calendar-min-date
Jan 8, 2026
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Document JapaneseCalendar.MinSupportedDateTime breaking change in .NET 11 Preview 1#50920
gewarren merged 6 commits intomainfrom
copilot/update-japanese-calendar-min-date

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Copilot AI commented Jan 7, 2026 •
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The Japanese Calendar minimum supported date changed from 1868-09-08 to 1868-10-23 in .NET 11 Preview 1 to align with Unicode CLDR data (CLDR-11375). This affects JapaneseCalendar.MinSupportedDateTime and invalidates dates in the 45-day gap.

Changes

  • Created docs/core/compatibility/globalization/11.0/japanese-calendar-min-date.md

    • Documents behavioral change affecting JapaneseCalendar.MinSupportedDateTime
    • Provides migration guidance for code using dates between 1868-09-08 and 1868-10-23
    • References Unicode CLDR update for historical accuracy

  • Created docs/core/compatibility/11.0.md

    • New index file for .NET 11 breaking changes

  • Updated docs/core/compatibility/toc.yml

    • Added .NET 11 section with globalization subsection

Example Impact

var calendar = new JapaneseCalendar();
var date = new DateTime(1868, 9, 15); // Previously valid

// .NET 11: Throws ArgumentOutOfRangeException
// Date falls before MinSupportedDateTime (1868-10-23)
calendar.GetYear(date);
Original prompt

This section details on the original issue you should resolve

<issue_title>[Breaking change]: Correct the Japanese Calendar supported minimum Gregorian date</issue_title>
<issue_description>### Description

The start date of the Japanese Meiji era has been updated from 1868-09-08 to 1868-10-23 to reflect the latest Unicode CLDR (Common Locale Data Repository) data and improved historical accuracy. This date also serves as the minimum supported date in the Japanese calendar.

Version

.NET 11 Preview 1

Previous behavior

JapaneseCalendar.MinSupportedDateTime previously returned 1868-09-08. Dates between 1868-09-08 and 1868-10-23 were also considered valid and supported by JapaneseCalendar.

New behavior

JapaneseCalendar.MinSupportedDateTime returns 1868-10-23 instead. Dates between 1868-09-08 and 1868-10-23 now considered invalid and unsupported by JapaneseCalendar.

Type of breaking change

  • Binary incompatible: Existing binaries might encounter a breaking change in behavior, such as failure to load or execute, and if so, require recompilation.
  • Source incompatible: When recompiled using the new SDK or component or to target the new runtime, existing source code might require source changes to compile successfully.
  • Behavioral change: Existing binaries might behave differently at run time.

Reason for change

To reflect the latest Unicode CLDR (Common Locale Data Repository) https://unicode-org.atlassian.net/browse/CLDR-11375 data and improved historical accuracy.

Recommended action

Update any code depending on the JapaneseCalendar.MinSupportedDateTime old value and avoid using Gregorian dates before 1868-10-23 with the JapaneseCalendar.

Feature area

Globalization

Affected APIs

JapaneseCalendar
JapaneseCalendar.MinSupportedDateTime

Associated WorkItem - 543457</issue_description>

Comments on the Issue (you are @copilot in this section)

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Internal previews

📄 File 🔗 Preview link
docs/core/compatibility/11.md Breaking changes in .NET 11
docs/core/compatibility/core-libraries/11/environment-tickcount-windows-behavior.md Environment.TickCount made consistent with Windows timeout behavior
docs/core/compatibility/globalization/11/japanese-calendar-min-date.md "Breaking change: Japanese Calendar minimum supported date corrected"
docs/core/compatibility/toc.yml docs/core/compatibility/toc

Add breaking change documentation for Japanese Calendar minimum date …
e42ce33 
…correction

- Create japanese-calendar-min-date.md breaking change article for .NET 11 Preview 1
- Create 11.0.md index file for .NET 11 breaking changes
- Update toc.yml to include .NET 11 section with Globalization subsection
- Document change from 1868-09-08 to 1868-10-23 as minimum supported date
- Reference CLDR-11375 and WorkItem 543457
Copilot AI changed the title [WIP] Update minimum supported Gregorian date for Japanese calendar Document JapaneseCalendar.MinSupportedDateTime breaking change in .NET 11 Preview 1 Jan 7, 2026
Copilot AI requested a review from gewarren January 7, 2026 02:04
@gewarren gewarren marked this pull request as ready for review January 7, 2026 22:30
@gewarren gewarren requested a review from a team as a code owner January 7, 2026 22:30
Copilot AI review requested due to automatic review settings January 7, 2026 22:30
@gewarren gewarren enabled auto-merge (squash) January 7, 2026 22:31
@gewarren gewarren requested a review from a team January 7, 2026 22:32
Copilot AI reviewed Jan 7, 2026
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Pull request overview

This PR documents a breaking change in .NET 11 Preview 1 where the Japanese Calendar's minimum supported date was corrected from 1868-09-08 to 1868-10-23 to align with Unicode CLDR data. The change updates the JapaneseCalendar.MinSupportedDateTime property value and invalidates dates in the 45-day gap between the old and new dates.

Key changes:

  • Creates new breaking change documentation for the Japanese Calendar date correction
  • Adds .NET 11 Globalization section to the breaking changes index
  • Updates the table of contents to include the new breaking change entry

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File Description
docs/core/compatibility/globalization/11/japanese-calendar-min-date.md New breaking change documentation describing the Japanese Calendar minimum date correction, including previous/new behavior, reason for change, and affected APIs
docs/core/compatibility/11.md Adds Globalization section to the .NET 11 breaking changes index with entry for Japanese Calendar change
docs/core/compatibility/toc.yml Updates table of contents to include Globalization subsection under .NET 11 with link to new Japanese Calendar article
docs/core/compatibility/core-libraries/11/environment-tickcount-windows-behavior.md Corrects title punctuation from hyphen to colon for consistency with other breaking change documents

BillWagner
BillWagner approved these changes Jan 8, 2026
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This LGTM. Let's :shipit:

@gewarren gewarren merged commit c7e29db into main Jan 8, 2026
17 checks passed
@gewarren gewarren deleted the copilot/update-japanese-calendar-min-date branch January 8, 2026 15:42
gewarren pushed a commit to gewarren/docs that referenced this pull request Jan 8, 2026
@gewarren
Document JapaneseCalendar.MinSupportedDateTime breaking change in .NE…
7da91bd 
…T 11 Preview 1 (dotnet#50920)
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[Breaking change]: Correct the Japanese Calendar supported minimum Gregorian date

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