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 1.34.0-wmf.10.

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.