From: Rabbi Moshe Sofer
To: Rabbi Wolf
A man whose parents died on the 29th of Adar Rishon in a leap year wishes to observe Yahrzeit on the 29th of Adar in regular years, while others argue it should follow a precise twelve-month count. The Chasam Sofer rules unequivocally that Yahrzeit follows the month's name—Adar—not elapsed days, demonstrating that a strict day-count would absurdly shift the Yahrzeit backward through the calendar year after year.
Source: Shu"t Chasam Sofer OC §14