OK, so let's assume that Gregorios's reform paralleled that of Gregory down to the exact date of the switch. *Here*, that was Friday, 1582 October 5 OS = October 15 NS. *There*, that would have been 1586 Υπερβερεταίο 11/21. Presumably they would have tried to get the Easter tables lined up with the actual full moon after the following spring equinox (1583/1587). According to the Gregorian epact table, 1583 has an epact of 7. That 7 represents the age of the moon on 1 January, which means that the nominal new moon was a week earlier on Christmas 1582, which corresponds to 2 Αυδυναίο 1587 *there*. So the WHATL epact for the year 1587 would be the age of the moon on Christmas Eve *here*, which was 29 (wraps on the new moon at 30=0). To verify, I applied the Alexandrian method manually. Christmas Eve in 1582 Gregorian was a Friday, so the first Κυριακό of the new WHATL year would be the 3rd of Αυδυναίο, making the Dominican(*) Letter C. Sure enough, using the Alexandrian computus on a WHATL calendar with an Epact of 29 and Dominican Letter of C yields the same date for the Paschal full moon (6 April = 12 Ξανθικό) and therefore Easter (10 April = 16 Ξανθικό). So now that I can do it manually I just need to derive the correct offsets to adjust for the general formula and then I can determine Easter for any WHATL year programmatically (and do analyses, such as find out exactly how often it coincides with Gregorian Easter). (*) So what's "Dominican" in TAKE? :) -- Mark J. Reed <[log in to unmask]>