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

Source?
I thought you had to calculate from the current day. If in the past 180 days you have stayed 90 days, then you need to leave today / you can't enter today.