Hello! This article seems to suggest that there was just an update put out earlier today that should allow payment date entries after 4/15. (The article itself was just modified about half an hour ago).
If it persists, you should be able to follow the helpful advice SweetieJean provided.
I am unable to test this on my end without paying for a test return, but I am going to try and hunt down some more information if possible. Thanks!
