1,280. All the Exceptions for Rosh Chodesh Tishrei

Hilchos Kiddush HaChodesh 7:5

In a year that follows a leap year, if the molad for Tishrei falls on a Monday, three hours and 589 chalakim or more past daybreak (2, 15, 589), Rosh Chodesh is not set for Monday but for Tuesday.

Hilchos Kiddush HaChodesh 7:6

If the molad of a regular year falls even one chalak earlier (3, 9, 203 or earlier – refer to halacha 7:4), then Rosh Chodesh is set for Tuesday (and not Thursday). If the molad of the year after a leap year falls even one chalak earlier (2, 15, 588 or earlier – see halacha 7:5, above), then Rosh Chodesh is set for Monday. Therefore, the way to calculate Rosh Chodesh Tishrei is as follows: first one determines the day of the week, the hour of the day and the number of chalakim in the hour when the molad will occur. The day of the molad will be Rosh Chodesh unless the molad falls on Sunday, Wednesday or Friday; the molad falls at or after noon; in a regular year, the molad occurs on the night of the third day after 204 chalakim of the tenth hour; or in a regular year that follows a leap year, the molad occurs on Monday after 589 units of the fourth hour. If the molad falls in one of these four ways, Rosh Chodesh is not set for that day. Rather, it is set for the next day or the day after, as has been explained.