Originally Posted by
elpiett
A curiosity: say you enter schengen on jan 1st and leave jan 2nd. If you come back April 2nd you can actually legally stay till sep 1st, as your 90 days ended on 30th of June and a new period started on July 1st. A ruling from the EU court that has given a lot of border control officers headaches already

Sounds untrue - total 90 days in running 180 days period. So if you came back 2/Apr the latest you should leave is 28/Jun supposing that you haven't spent any days prior to 1/Jan (first entry)