A year is a leap year if it's divisible by 4 — except century years, which must be divisible by 400.

• 2020 → ÷4 → leap. ✓
• 1900 → ÷100 but not ÷400 → not leap.
• 2000 → ÷400 → leap. ✓
• 2023 → not ÷4 → not leap.

This matters because leap years change the January and February doomsday dates (and the ⌊YY/4⌋ term already counts the leap days for the year calculation).

Split the last two digits with 12: doomsday = anchor + q + r + s (mod 7), where q = YY÷12, r = the remainder, s = r÷4.

For year 19YY:

1. q = how many whole 12s in YY.
2. r = what's left over (YY − 12q).
3. s = r ÷ 4 (whole part).
4. Add q + r + s to the century anchor, mod 7.

This small sum captures how far the doomsday has drifted since the century's start.