How to replicate the issue:
Joomla 4
PHP 8.1 or 8.2 - (I'm on 8.2).
Switch on error-reporting.
Edit an 'Event' type article and access the Event Info panel.
That panel has 2 Calendar types and the warning shows for both fields.
How to fix it
Despite the warning pointing to a core Joomla file, the problem is in the creation of the field in the template XML file.
Edit file: /ja_charity/contenttype/types/event.xml
and add translateformat="true" OR filterformat to both fields ctm_start and ctm_end.
Fix documentation
I'm not taking credit for it, just helping and sharing it lol
https://github.com/joomla/joomla-cms/issues/42790
Thank you