When was Yitzchok supposed to be born?

Question

In Parshas Vayeira, there is the Besurah of the birth of Yitzchok in the coming year:

לַמּוֹעֵ֞ד אָשׁ֥וּב אֵלֶ֛יךָ כָּעֵ֥ת חַיָּ֖ה וּלְשָׂרָ֥ה בֵֽן

This means exactly one year later to the day, as Rashi proves from this that it was Pesach, since Yitzchok was born on Pesach.

Yet, in the Parsha of Bris milah, at the end of Parshas Lech lecha, it also says that exactly one year later there will be the birth of Yitzchok:

וְאֶת־בְּרִיתִ֖י אָקִ֣ים אֶת־יִצְחָ֑ק אֲשֶׁר֩ תֵּלֵ֨ד לְךָ֤ שָׂרָה֙ לַמּוֹעֵ֣ד הַזֶּ֔ה בַּשָּׁנָ֖ה הָאַחֶֽרֶת

This took place three days earlier as Rashi says in parshas Vayeira:

אמר רבי חמא בר חנינא יום שלישי למילתו היה, ובא הקדוש ברוך הוא ושאל בשלומו:

I had a thought that maybe the year is being calculated differently in each case, as the chizkuni brings:

חזקוני בראשית פרק יז פסוק כא למועד הזה בשנה האחרת אמר רב הונא בשם רב אידי אותה שנה מעוברת היתה פי' ממועד למועד הוי שנה אחת והוא קורא אותה שנה אחרת מפני חודש העבור שנתוסף בה.

This would indicate that the words Shana Acheres refer to the fact that the year was a leap year, and in Parshas Vayeira these words dont appear. This lead me to think that perhaps in Lech Lecha the "year" is Lunar, and in Vayeira it is Solar, but i still can't work out the numbers, any ideas?

Answer 1

I found that the Mizrachi (Beginning of Parshas Vayeira) addresses my question, including all the sources which I mentioned, adding the chazal that "שסרט לו סריטה בכותל דלשנה אחרת כשתבא חמה לכאן תלד שרה" ; further enforcing that the promise of Yitzchok's birth was for exactly a year later, to the day. The Mizrachi concludes that there are obviously אגדות חלוקות.

He answers that it is consistent with Rashi's style to quote conflicting Midrashim, and therefore there is no problem. Alternatively he suggests that when Rashi writes כעת חיה - פסח היה he means the third day of Pesach.

This, however does not explain the Midrash itself that says that this was three days after the bris mila, since it would turn out that on two separate days it was said that in exactly a year there would be the birth of Yitzchok.

This can be explained with the words of Rashi: למועד - לאותו מועד המיוחד שקבעתי לך אתמול למועד הזה בשנה האחרת. This seems to mean that they were referring back to the original promise of three days earlier, so there is no conflict after all.