Date: 2 Adar 5574
From: Rabbi Moshe Sofer
To: Rabbi Shalom
A widow subject to the twenty-four-month nursing restriction asked whether she must wait through the extra thirteenth month of a leap year. The Chasam Sofer argues that once approximately 732 days (two solar years) have elapsed — roughly twenty-two days into the thirteenth month — the natural nursing requirement has been met, and since the principle of la'Kel gomer alai operates only for the benefit of Torah-observers and not to their detriment, remarriage is permitted without hesitation during the final week of the thirteenth month.
Source: Shu"t Chasam Sofer EH I §137