How do leap years change the method in practice?

Only the January and February anchor dates shift by one: Jan 3 → Jan 4, and Feb 28 → Feb 29. Everything from March onward is unchanged.

Because the leap day is added at the end of February, dates before it move:

• January doomsday: 1/3 in common years, 1/4 in leap years.
• February doomsday: last day — 2/28 common, 2/29 leap.

March through December (4/4, 6/6, Pi Day, "9-to-5 at 7-11", etc.) never change. So for a Jan/Feb date, first ask "is this a leap year?" and pick the right anchor.