How to Get Faster at the Doomsday Algorithm

This guide is for people who already get the right answer and want it sooner. If you are still learning the method, read the full walkthrough and the memory tricks first — everything below assumes the anchors, the doomsday dates and the year step are already in your head.

Where the time actually goes

Time a few rounds and a pattern appears. Recalling the century anchor and the month’s doomsday date takes well under a second each. Almost all of the rest goes into two places: turning the year into a number, and counting from the doomsday to your date. Getting faster means shrinking those two steps — and not redoing work you have done before.

For reference, the main game rates an answer as fast under six seconds and the XP speed bonus tops out under five; experienced solvers get comfortably under that. Your stats page charts your time per round, so you can see which of the ideas below actually moves it.

Think in numbers, not names

Hold the whole calculation as digits from 0 to 6 and convert to a weekday name once, at the very end. Every translation between “Wednesday” and “3” in the middle of the working is a small stall, and there are several of them per date.

Conway did this with nicknames — Noneday, Oneday, Twosday, Treblesday, Foursday, Fiveday and Six-a-day — and the joke is the point: the name and the number become a single thought. The Weekday number drill exists for exactly this. On the play page you can answer with the number keys, so the conversion at the end disappears too.

Do the cheap half first

Read the date and take the month-and-day offset before you touch the year. That part is pure recall: the month’s doomsday date, the distance to your day, cast out sevens. Now you are holding one small number while you do the only real arithmetic, instead of holding the year’s doomsday while you hunt through the month.

Then count the short way around. Six days after is one day before; five after is two before. The count is never longer than three in either direction.

Know every month’s smallest doomsday

The doomsday dates you memorized are one doomsday per month. Every month has four or five of them, seven days apart, and the useful one is the smallest — the first doomsday in the month, counting a “0th” as the last day of the month before. The offset for any date is then just the day number minus that, with the sevens thrown away.

January and February move by one in a leap year — the 3rd becomes the 4th, and February’s “0th” becomes the 1st. Everything from March on is fixed, which is why March is the easiest month: its offset is simply the day number, mod 7.

Reduce the day first if it is big: the 26th is 5, the 31st is 3. With both numbers small the subtraction is instant, and a negative result just means “count back.”

Memorize the doomsday years

Fourteen years in every century have a doomsday equal to the century anchor — their year code is 0. Learn them and most years become a short walk instead of a calculation:

00, 06, 17, 23 — 28, 34, 45, 51 — 56, 62, 73, 79 — 84, 90.

The gaps repeat: +6, +11, +6, +5, and then again. That is the calendar’s 28-year cycle, so once you know the first four the rest follow.

From the nearest doomsday year, step forward one for each year, and two for a year that is itself a leap year (its doomsday falls after February 29th, so it feels the extra day). Stepping back, undo the same amounts: subtract one, or two if the year you are leaving is a leap year.

• 1966. The nearest doomsday year below is 62. Then 63 adds one, 64 adds two, 65 one, 66 one: five in all. Wednesday plus five is Monday.
• 2027. Closer from above: 28 is a doomsday year, and it is a leap year, so stepping back from 28 to 27 subtracts two. Tuesday minus two is Sunday.

This is also the fastest fix for the years you meet most. If the last decade or two is where most of your dates live, walk each of those years once from a doomsday year, and after a few repetitions you will know them outright. The Doomsday of a year drill is the place to do it.

Know the current decade cold

Whatever else you memorize, know the doomsday of this year and the years around it without thinking. Recent dates come up far more often than any others, and for them the whole year step collapses into a lookup. The 2020s:

Ten weekdays, and every date from 2020 to 2029 is one memorized value plus a month offset. Add your own birth year and the birth years of the people who ask you — the Your dates drill lets you practice exactly those.

Shave the year step itself

For years you have not memorized, you still calculate. One thing makes each route quicker:

• With Conway’s dozens: learn the twelves table as year codes. Each dozen adds exactly one — 12 → 1, 24 → 2, 36 → 3, 48 → 4, 60 → 5, 72 → 6, 84 → 0, 96 → 1. So for 1985 the dozen (84) is already 0 and only the remainder matters: 1 plus its quarter (0) gives 1. You skip counting the twelves and the addition that follows.
• With odd + 11: the halving is the only real work, and it is fastest when you never say the number to yourself — watch the parity, act, move on. It also hands you the leap year for free: if you never added 11 at either pass, the year is divisible by 4.

Which route is faster for you is an empirical question. Play twenty rounds with each — the year method setting switches the hints and drills to match — and let the clock decide.

Only check leap years when it matters

The leap-year test matters for January and February dates only; for anything else, skip it entirely, since the doomsday dates from March on are the same in every year. When you do need it, look at the last two digits: if the tens digit is even, the year is a leap year when the units digit is 0, 4 or 8; if the tens digit is odd, when it is 2 or 6. The century exception only ever bites on a year ending in 00.

Habits that cost seconds

The slow habits are hard to notice from the inside. Three worth checking:

• Narrating. Saying the steps in an inner voice caps you at speaking pace. Practice letting each step be a silent move: see 66, see 5.
• Re-checking. Verifying a step you have done a thousand times doubles its cost without improving it. Commit, answer, and let a wrong answer teach you — the game shows you exactly which step slipped.
• Restarting. If you lose a number, do not go back to the beginning; recompute the step you lost and carry on. Keeping fewer numbers in your head — one offset, one running year value — is what makes losing one rare.

Drills for speed rather than accuracy

Accuracy drills are untimed and ask whether you can. Speed drills add a clock and ask how fast, and the difference matters: a step only becomes automatic once you have done it under time pressure many times.

• Putting it all together (hard) — full dates against the clock, the closest thing to the real game.
• Year code (odd + 11) and Doomsday of a year — the year step on its own, where most of the time goes.
• Flash mode — the date shows for as little as half a second and then vanishes. It forces you to take the whole date in one look and hold it, which is the skill that stalls most people once the arithmetic is quick.
• Arena — against another player, where correct beats wrong and speed breaks the tie. Nothing sharpens a step like a rival.

Ten minutes a day of these beats an hour at the weekend. Track the average time on your stats page, and expect it to fall in steps rather than smoothly: each shortcut above removes a whole chunk at once.