Is a 12-month open access mandate compatible with a 12-month embargo period?
You might be able to ask your librarians to help you with this. For example, I know the University of Cambridge's have a program with which you can submit your work after which they'll take care of checking the requirements.

Given that you're at most one day in the wrong, however, I wouldn't worry too much - you're definitely working in the spirit of the rules.