Simplest terms, The first resurrection will be on the first day of the 7th Biblical month, Tishri, of some unknown year (Yom Teruah, the feast which we can’t know when it will happen until the new moon shows up shortly after sundown, which is its beginning. All other feasts are somewhere in the middle of the month, and are thus known well before they happen)
If one counts back the days from the end of the sixth month, it is found to be Purim, which is on the 14th day of the 12th month, Adar, 3.5 years earlier. If any Adar Bets were to occur in that time frame, it would not work out, so we do know that the first resurrection will have to occur in a period beginning the second year after an Adar Bet has happened.
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Thank you for the reply.