The exact timeline of the Flood is complex and I will surely confuse and bore readers if I delve to deeply into it (see here for a discussion but be forewarned). However, an interesting point emerges from Ramban’s (Bereishis 8:4) explanation of the Flood chronology. Ramban’s method of reconstructing the timeline assumes months of 30-day lengths, i.e. solar months and not lunar months.
It seems that Ramban assumes that the calendar followed a solar cycle rather than the lunar cycle which is currently in use. In other words, before the Torah was given, or before the commandment to utilize lunar months was given in Egypt (Shemos 12:2), the de facto calendar was solar. Perhaps one could say that the natural calendar is solar while the Torah calendar is lunar. This corresponds to what explained in an earlier post that natural days are from sunrise to sunrise while Torah days are from sunset to sunset. When the Torah was given, the ways of measuring the calendar was changed.
This view of the Ramban that pre-Torah months were solar is confirmed by the Mekhilta De-Rashbi and Midrash Ha-Gadol on Shemos 12:2: “‘This month is for you’ – The forefathers did not count from it.” Similarly, the Mekhilta (De-Rabbi Yishma’el) says on that verse: “‘This month is for you’ – Adam did not count from it.” The late Midrash Sekhel Tov on Bereishis 34:25 states explicitly that solar months were used until God commanded us to use lunar months. (For a fuller discussion of this issue, see Torah Shelemah, vol. 13 ch. 1.)