How they will know went you enter spain
Some one posted that if you arrived in spain before 1st jan they will not know what date you arrived so you can stay longer than 90 days they might not know the exact date but if there is no record of you entering spain it’s obvious that you arrived before the checks started hence they will know you arrived before jan 1st so you have to leave by 31st March as it’s simple for them to count back I don’t know if you get fined if you leave over the 90 days