Is a 12-month open access mandate compatible with a 12-month embargo period? The conditions of my research grant say that I must ensure open access to the peer reviewed publications describing project results by making them available in a repository "within twelve months of publication" (the accepted manuscript is enough).

Springer's conditions for the embargo period say "Authors may self-archive the author’s accepted manuscript of their articles on their own websites. Authors may also deposit this version of the article in any repository, provided it is only made publicly available 12 months after official publication or later."

Are these two things compatible? My understanding is that they are, because the day that comes exactly twelve months after publication is both the last day that is "within twelve months of publication", and the first day that is "12 months after official publication or later", so by making the publication available exactly that day I would be complying with both the funding agency and the publisher. But not being a native English speaker, I'm not 100% sure.
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.