For Pref > Oracle > Options > Use..., the checkbox is not checked.
For Pref > U/I > NLS > Date, the select is User Defined with a mask of DD-MMM-YYYY. MMM isn't a valid Oracle format string, but maybe that's not what is being used here. Oracle would want this to be MON. I have not recently changed this. I ran PL/SQL Dev 14, and the settings are the same there, but the displayed date/times in the scheduler are processed without error in that version. Here is a sample Start Date from PLSQLD 14 (and 15): 07-Sep-2021 01:35:06 PM.
In PLSQLD 15, when I enable Pref > Oracle > Options > Use Timestamp format, the Start Date appears as: 07-SEP-21 01.35.06.000000 PM -04:00, and is processed without error. However, when I use this format, there doesn't seem to be a date picker available.
If i clear Use Timestamp Format, and use the Windows formats for Date and Time, the Start Date is presented as: 9/7/2021 1:35:06 PM. The date picker is available, but it does not provide a time picker interface.
If I use User Format of DD-MON-YYYY, the Start Date presented is 07-9O35-2021 01:35:06 PM (which is clearly messed up, so maybe the MON is not what it is expecting at that point).
At this point, use of the Windows formats seems to work the best, so I will stick with those.