Short version:
Teams desktop: Date/time format matches Mac and Windows OS language settings. Your date/time formats in Teams will match the Mac and Windows OS regional settings on desktop. Previously, Teams would only show date/time in the format corresponding to the application language. This feature rollout will begin mid-April and should be complete by late April 2021

Details:
With this update, your date/time formats in Teams will match the Mac and Windows OS regional settings on desktop. Previously, Teams would only show date/time in the format corresponding to the application language.

If you have regional settings set to a language different than the application language, you date and time formatting will reflect the appropriate date/time format for your OS regional setting.

Note: Only the Gregorian calendar is supported, regardless of the OS calendar setting.

Availability:
This feature rollout will begin mid-April and should be complete by late April 2021.