The Eastern Christian churches still determine the Easter dates using the older Julian Calendar method.The statement that Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox, is only an approximate statement of the actual ecclesiastical rules.
The new calendar was issued in February in the papal bull called "Inter gravissimas".
This new calendar is referred to as the Gregorian Calendar.
Of the incidents last year, the worst involved Iranian forces capturing 10 American sailors and holding them overnight.
It became a propaganda coup for Iran's hard-liners, as Iranian state television repeatedly aired footage of the Americans on their knees with their hands on their heads.
The episode marks the latest in a series of close encounters between US ships and Iranian naval vessels.