When we add the words "Ulichaporas Pasha" during Musaf of Rosh Chodesh when it's a leap year, when is the year determined - Nisan or Tishrei?
The sefer אליהו רבה Siman 423 writes that it should be said for all the months of the leap year, but the custom is to say it only until Rosh Chodesh Nisan. The Pri Megadim explains that the Eliyahu Rabbah means that if not for our custom to stop after Adar we should really continue saying it until Tishrei. (The מהרי"ב explains that our custom of stopping after Adar is because these words were added in a leap year in order that people will realize that it is a leap year, and therefore after the second Adar there is no longer a purpose to this addition.)
According to this we add the words ולכפרת פשע from Tishrei.
As far as I have heard, the Yekke custom is to only say it in Adar Bais, as until then we wouldnt know that its a leap year, and afterwards its the next year - Hachodeh haze ROsh chodoshim