From: Rabbi Moshe Sofer
To: Rabbi Avraham
The Chasam Sofer rules that the leap month (chodesh ha'ibur) is included in the definition of a 'year' under Rabbeinu Tam's takkanah governing return of dowry and tosefes kesubah. Since a complete year means a full calendar cycle — which in a leap year spans thirteen months — the two-year period in this case had not yet elapsed at the husband's death, and the widow therefore receives only half the tosefes kesubah.
Source: Shu"t Chasam Sofer EH I §119