Commentary for Rosh Hashanah 4:1

קמ"ל דניסן ר"ה למלכים ויום אחד בשנה חשוב שנה ואם לא עמד אלא באחד בניסן אין מונין לו שנה עד שיגיע ניסן אחר פשיטא

This teaches us that Nisan is the New Year for kings, and that one day in a year is reckoned as a year.' But if he ascended the throne on the first of Nisan he is not reckoned to have reigned a year till the next first of Nisan comes round'. This surely is self-evident? - It had to be stated in view of the case where his election to the throne was determined upon<span class="x" onmousemove="('comment',' By the notables of the State. Lit. 'they (i.e., their votes) have been counted for him'.');">¹</span>

Rashi on Rosh Hashanah