That's fixed it.
But it raises a philosophical question. If a feature has been broken for almost three years before anyone notices, do you fix it, or do you remove it?
It's certainly a question worth asking. Now, the user experience isn't complicated by these fields being editable, so I don't see any downside by allowing this ability. Also, one contributing reason why we haven't got bug reports on this could be that it seems to work - no error messages. It's only when you revisit the same image you see that the date change was ignored.