How did Avraham set the calendar? Was it fixed like Hillel II made or was it based on sightings? Did he add an Adar every few years? Related question: When the Torah says "years" like in Adam lived x number of years, are those Jewish years or solar years? Related question: When chazal tell us that the Mabul happened on such and such a date in the Hebrew calendar, does that mean a calendar existed like Hillel's before matan torah? What sources can you point me to that answer these questions?
Menachem Weiman, St. Louis, MO, USA
1) Pirkei d'Rebbi Eliezer, chapter 8, tells us that the sun and moon were created on 28 Elul. Years, months, days and nights and hours were in front of ha'Kadosh Baruch Hu, who added an extra month to the years. Afterwards, Hash-m handed this on to Adam in Gan Eden. Adam passed it on to Chanoch, who entered into the "Sod Ha'Ibur," the secret of adding the extra month. Chanoch passed on the secret to Noach, Noach to Shem, and Shem to Avraham.
2) Solar years can also be Jewish years. The Gemara (Kerisus 6a) tells us that the incense contained 368 Maneh -- 365 Maneh corresponding to the days of the solar year, and 3 extra Maneh for Yom Kippur (some people recite this every day in "Pitum ha'Ketores"). The Gemara in Rosh Hashana (6b) states that according to Rebbi, the "complete year" referred to in Vayikra 25:30, is 365 days. Rabbi Akiva Eiger writes in Teshuvos 3:12 that according to Rebbi a year is always considered to be 365 days.
Therefore, according to Rebbi we can say that Adam lived 950 solar years, while according to the other Tana'im we can say he lived 950 lunar years.
3) According to Pirkei d'Rebbi Eliezer that I cited above, we can say that the most accurate calendar possible was already known in the earliest generations.
Kol Tuv,
Dovid Bloom