Using a modification of the Gauss algorithm to calculate Orthodox Easter date
For reference, the description of the algorithm given below the calculator is taken from Wikipedia
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Description of Orthodox Easter date calculation
To define the old style Orthodox Easter date you have to:
Divide the year number by 19 and define the remainder of division a.
Divide the year number by 4 and define the remainder of division b.
Divide the year number by 7 and define the remainder of division c.
Divide the sum 19a + 15 by 30 and define the remainder d.
Divide the sum 2b + 4c + 6d + 6 by 7 and define the remainder e.
Define the sum f = d + e.
If f ≤ 9, then Easter will be celebrated on March 22 + f March; if f > 9, the Easter will be celebrated on f - April 9th.
For a new style conversion, you have to move forward by 13 days in the 20th and 21th centuries. Orthodox Easter in the 20th and 21st centuries takes place from April 4th to May 8th.