The strftime, and gmstrftime functions have been deprecated and will be removed in 9.0.
See https://php.watch/versions/8.1/strftime-gmstrftime-deprecated for further information.
We will need to get on this ASAP and should take this opportunity to improve the core_date class (and/or replace it if relevant).
The recognised replacement is to use the Intl library, which uses Unicode CLDR data rather than system locale data and alleviates other issues.
With the right API design we can also hopefully improve our testing integrations.