WHERE DO WE LEARN THIS FROM? IS IT MIN HATORAH OR MIDRABBANAN?
Michael Plaskow
Netanya, Israel
The Gemara cites this rule many times, but it never brings a source. It appears to be based on "Sevarah," logic.
This Halachah is definitely mid'Oraisa, since the Gemara applies it to Nazir (in Nazir 6a) and tries to apply it to Zavah as well (in Nidah 33a).
D. Zupnik
This Halachah might be loosely related to the Gemara's teaching that "a one day old cow is called a "Par" (Bava Kama 65b), which is indeed learned from a verse (ibid.).
(We also find that a related Halachah that dictates one day (or according to others, 30 days) in a year is called a year (Rosh Hashanah 10a); this too, does not have an explicit source.)
M. Kornfeld
Proofs of it being mid'Oraisa (as well as it's application) are discussed in the Sdei Chemed Kuntres Hakllolim Mareches Hamem Kllal 104.
Regarding "a one day old cow...," IMHO over there Pshat is that from birth "Ki Yivoleid" it is a Shor, not dependent on it being a day old.
Perhaps as a source for the Limud we can use the Mechilta quoted by Rashi Shmos 21:21 especially according to the Nusach as quoted in the Mizrachi on the Rashi "Sheuroi Yom Vhu Shnei Yomim Miktzosoi Mizeh Umiktzosoi Mizeh," note also the Targum YBU "Yomin Kti'in."
Kol Tuv,
Yitzchok Zirkind