When I add 87 days to Jan 1, 2009 00:00, I get the result: March 29, 2009 00:00, but when I add 88 days to the same date, I get March 29, 2009 01:00. Why?
It is because of the Daylight Saving Time mechanism. We have the reverse picture when the calculation result passes over the date when we set clocks back to one hour (the so-called, «winter-time»).