I was going to say check if the server time zone or clock was moved, but this is O365. Was the site collection region settings changed, or anything ancillary changed. Your pc settings, or your profile setup. Just want to check the obvious before looking at other things.
I actually found the source of the problem after trying several input values. Only 1 "Due Date" was causing the problem, e.i 31 March 2019. After investigating this date, the problem is now obvious since at this date in 2019 the summer timing will start.
I simply changed the Due Date of such records to April 1, and all is working now.
Thanks