Manually changing entry date fails

[9R]

Setting the date of an entry manually to a different date than the current date is not saved properly.

oc-version: 6.0.3 (stable)
journal-version: 0.4.1

When I add a new entry and try to set the date to some other date and time in the past this change is not saved properly. The next time I reload the page the entry shows its date and time of creation again.
Also it is not possible to save entries without time and date only. Unchecking "include time" seems to be ignored and current time is saved.

Same issue when I try to edit date & time of an entry that was created earlier.

Let me know if I can supply further debug information.