An upgrade to the timestamp library used by MediaWiki is resulting in two
changes to the handling of timestamp inputs to the action API. There will
be no change to timestamps output by the API.
All of these changes should be deployed to Wikimedia wikis with
Historically MediaWiki has ignored timezones in supported formats that
include timestamps, treating them as if the timezone specified were UTC. In
the future, specified timezones will be honored (and converted to UTC).
Historically some invalid formats were accepted, such
as "2019-05-22T12:00:00.....1257" or "Wed, 22 May 2019 12:00:00 A potato".
Due to improved validation, these will no longer be accepted.
Support for ISO 8601 and other formats has also been improved. See
https://www.mediawiki.org/wiki/Timestamp for details on the formats that
will be supported.