Datetimes with IST timezone are broken

Dates entered with the datetime picker using a time zone of "IST" are not displaying properly in posts. They look ok in the editor preview, but they break in the rendered page, throwing a red banner for site admins and showing the time as though it were UTC instead. Things all work ok if I use the IANA Asia/Kolkata tz.

Here’s the reproducer:

[date=2024-01-18 time=03:00:00 timezone="IST"]2024-01-18T03:00:00Z
[date=2024-01-18 time=03:00:00 timezone="Asia/Kolkata"]2024-01-17T21:30:00Z

Displays as:

image

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