Important: #77702 - Custom render types for date and datetime fields must use ISO-8601 ¶
See Issue #77702
Historically, TYPO3 used its own special, localized formats for passing date and datetime values between server and client. To get rid of any possible problems with that, we now use ISO-8601, a standard format for date/time representations.
Due to that, you need to adapt your custom FormEngine render types if you use them for any date/datetime fields, even those stored as integers in the database (eval=date/datetime).