DOOMSDAY Logs

2023 → Tuesday. Century anchor for the 2000s = Tuesday (2). YY = 23: q = 23 ÷ 12 = 1 r = 23 − 12 = 11 s = 11 ÷ 4 = 2 Sum: 2 + 1 + 11 + 2 = 16, mod 7 = 2 = Tuesday. Check: 2023's doomsday is Tuesday. ✓ (4/4/2023 was a Tuesday.)

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

Each common year moves the doomsday +1 and each leap year +2, so the drift is YY + (leap years) = YY + YY÷4 — and the 12-split is just an easy way to compute that mod 7. A normal year is 365 = 52·7 + 1 days, so the calendar shifts one weekday; a leap year shifts two. Over YY year...

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

It's another route to the same year's doomsday; use whichever feels faster — both give identical results. The "odd + 11" method: if YY is odd add 11, halve it, if that's odd add 11 again, take mod 7, and subtract from 7. It reaches the same answer as the 12-remainder-quarter meth...

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

A doomsday for each century's "00" year: 1800s → Friday, 1900s → Wednesday, 2000s → Tuesday, 2100s → Sunday. Century Anchor # 1700s / 2100s Sunday 0 1800s / 2200s Friday 5 1900s / 2300s Wednesday 3 2000s / 1600s Tuesday 2 The pattern repeats every 400 years (the G...

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

2000s = Tuesday ("Y2K → Twos-day"); 1900s = Wednesday. 2000s → Tuesday: the year 2000 doomsday was a Tuesday — think "Y2K, Twosday" (2 = Tuesday's number). 1900s → Wednesday: the workhorse anchor for 20th-century dates. From those two you can reach the others via the 400-year c...

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

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

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

1978 → Tuesday. Century anchor for the 1900s = Wednesday (3). Now YY = 78: q = 78 ÷ 12 = 6 r = 78 − 72 = 6 s = 6 ÷ 4 = 1 Sum: 3 + 6 + 6 + 1 = 16, and 16 mod 7 = 2 = Tuesday. Check: 1978's doomsday is indeed Tuesday. ✓ (So 4/4/1978, 6/6/1978, etc. were all Tuesdays.)

Quiz: DOOMSDAY / Year & Century Anchors | updated: Jul 30, 2026

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...

Quiz: DOOMSDAY / Full Method & Practice | updated: Jul 30, 2026

February's doomsday is the last day of February (2/28, or 2/29 in a leap year); January's is the 3rd, bumping to the 4th in a leap year. Both land on the same weekday, so they move together. February: the last day — 2/28, or 2/29 in a leap year. It is the leap boundary, so there...

Quiz: DOOMSDAY / The Doomsday Dates | updated: Jul 30, 2026

Century anchor → year's doomsday → count from the nearest doomsday date to your target. Century anchor — a memorised weekday for the century (2000s → Tuesday, 1900s → Wednesday). Year's doomsday — adjust the anchor by a small calculation on the last two digits of the year. The d...

Quiz: DOOMSDAY / What Is Doomsday | updated: Jul 30, 2026