The old date time formatter allowed setting the attribute to a default value if it could not be converted. This was useful as it allowed you to set to a null value or dummy date. The datetimeformatter just passes all features with invalid dates out the rejected port, if formatting a number of attributes with dates there is no information as to which attribute has the invalid date.
Released
Improvements to the DateTimeConverter
Rejected features also seem to loose the original data - it would be helpful to retain these so that further work/ manipulation on the data could be done.
+18- Safer
In FME 2023 and newer using the latest version of the DateTimeConvertor should address a few of these: the rejection message should now indicate what attribute could not be converted and the original attribute values are maintained