Mentioned as a missing property in issue #1829
Discussions in the TalentSignal W3C community group suggest two properties are necessary to resolve this:

jobStartDate

Definition: The date on which a successful applicant for this job would be expected to start work. Choose a specific date in the future or use the jobImmediateStart property to indicate the position is to be filled as soon as possible.
Expected type: ISO 8601 Date Text

jobImmediateStart

Definition: An indicator as to whether a position is available for an immediate start.
Expected type: Boolean