Date of Armenian Christmas celebration

Why is Christmas celebrated on Jan 18th in Jerusalem?

Here is info form the newsletter of St.Vartan cathedral:

"...On January 18 and 19, members of the St. James Brotherhood took part in the celebration of the Nativity and Theophany of Our Lord Jesus Christ in Bethlehem. (Christmas is celebrated later in the Holy Land because its date is calculated based on the Julian, rather than the more modern Gregorian, calendar.)..."

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