A Mess of Easters
You'd think calculating Easter could be simple. Well, it sort of is for the western denominations. For them, Easter is the first Sunday after the first full moon after March 21 (which is used instead of the true astronomical equinox, which can also be on the 22d of March, and varies depending on location--note that it would be politically incorrect to say the "Spring Equinox" because this ignores the southern hemisphere, where March is fall). March 21st is located in time by using the new Gregorian calendar, which spread from western continental Europe to the UK and then farther, and since 1922 is used universally.
Not quite universally, though. The calculation of the eastern Easter is based on the March 21 of the old Julian calendar, despite that nobody uses it (well, someboby must be using it, those who calculate the eastern Easter). Furthermore, according to some the eastern Easter has an additional requirement. It must occur later than Passover. Thus, the eastern Easter requires us to find the first full moon after roughly April 4th and postpone it until after Passover. This "rule" has not found any application because from the year 783 and on, Passover precedes the eastern Easter. For more details go here, from where this calculator were adapted.
Anyhow, here is the calculator. Its reliability is reflected in its price, if you incur major expenses in reliance on this and your plans fail, you will not have a claim against anyone.