From: Rabbi Moshe Sofer
To: Rabbi David
Two questions are addressed: whether Pidyon HaBen may be performed on day thirty once the astronomical month has elapsed, and the meaning of Rabbeinu Gershom's cherem regarding disrupting prayer services. The Chasam Sofer rules that Pidyon HaBen must wait until day thirty-one, since we count days not hours for months.
Source: Shu"t Chasam Sofer OC §81