הלכה על מגילה 9:14

The laws of the commandment - for example, interrogating [those who offer] testimony concerning the new month; instilling fear in the witnesses on occasion; the law concerning circumstances under which the Shabbat may be desecrated for this testimony; for what [considerations] we do or do not intercalate; which month they would intercalate, i.e. Adar, as they, may their memory be blessed, expounded, "'You shall observe this commandment in its proper time' (Exodus 13:10), this teaches that we only intercalate at the time near the holiday"; and they, may their memory be blessed, further expounded on this verse, "From where do we know that we only intercalate the month during daytime? [We learn this from the] verse, [which] states, 'from year to year' (yamim yemima, literally from day to day) (Sanhedrin 10b); and they, may their memory be blessed, further expounded, "'For the months of the year' (Exodus 12:2) - it is months you calculate towards the year, not days" (Megillah 5a); furthermore did they say on this matter, "'A month of days' (Numbers 11:20), it is days you calculate towards the month, not hours" (Megillah 5a); and the rest of its details - are [all] elucidated in Tractate Rosh Hashanah, and in the first chapter of Sanhedrin, and similarly in Berakhot (see Mishneh Torah, Laws of Sanctification of the New Month 1).