Illustrative example
A month-to-month tenant upstate gives three weeks' notice that she is leaving at the end of the month. The landlord says he is entitled to a full month and wants the extra rent.
Outside New York City a monthly tenancy may be terminated on notice given at least one month before the expiration of the term, and the tenant is expressly among those who may give it. So this turns on the arithmetic rather than on anyone's reasonableness: three weeks is short of the month the section asks for.
They split it: the tenant pays the extra week and leaves the apartment clean and empty on the earlier date so the landlord can show it immediately.