How do you turn the century anchor into the year's doomsday (the "12-remainder-quarter" method)?

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.