Rabbi Yitzchak, who knew the Keitz, calculated in the years of Malchut. He said: The world was created ששת ימים, which is six times one thousand years, from year 1 to year 6001. Each day is Erev and Boker, and so the world is 12 times 500. But the very end of Malchut is a little later, as is written (Yirmyahu 31:16) יש תקוה לאחריתך. Therefore, the very end must be the year 6133, 511 (תקו"ה) times 12 from year 1.
Rabbi Yehuda, who heard from Eliyahu (Sanhedrin 97b) and knew Ma'aseh B'reishit, calculated in years of the Earth. He said: It is written (B'reishit 6:3) "his days shall be a hundred and twenty years." Read "his generations". The generations are ע"ב נ"ב, Keter Malchut. That is, 72 generations of 1000 years until the Mabul, and 52 generations of 120 years thereafter. Also, the generations are Tzaddik Malchut, צ' נ"ב, Ninety generations of 800 years until the Mabul, followed by 52 generations of 120 years until the end, in accordance with what is written (B'reishit 7:1): צדיק לפני בדור הזה. Until Yetziat Mitzrayim are ברי"ת generations of 120 years. The Brit of the Tzaddik, Noach. After Yetziat Mitzrayim, מ' דורות remain, forty generations of 120 years. Together: ברי"ת מ' דורות, or 78,240 years, until the year 6129. The Brit of the Mem.
Replied Rabbi Yitzchak: Add four years and get ברי"ת ד"ם.
Rebuts Rabbi Yehuda: Even so, ד"ם is Dalet Mem, not Mem Dalet. Add Dalet to the Rosh and not to the Keitz. Say ד' שנים and afterwards מ' דורות. Derive that Yetziat Mitzrayim was in the year 1325 and agree with Chachamim that Bayit Rishon stood 410 years!
