One day, five counters

The same day, as five different machines count it. ← Calendar Contraptions

Today's date is not one number. Ask five systems what day it is and you get five answers — all of them correct, none of them agreeing, each counting for its own reasons. Pick a date and watch what each counter makes of it.

Three clocks that refuse to fit

Every calendar is a treaty between three clocks that will not be reconciled. The day is set by the Earth's spin. The moon takes about 29.53 days to orbit — twelve of those make 354 days, eleven short of a year. And the year itself takes 365.2422 days, which no reasonable number of whole days divides into. A solar calendar hides the moon; a lunar calendar lets the seasons wander; something always gives. Each counter below is a different choice about what to hide — and each one breaks somewhere interesting.

The counter that never argues

The Julian Day Number is what you get when you give up on months, years and politics entirely: just count the days. One integer per day, increasing by one forever. The scheme was proposed in 1583 by Joseph Justus Scaliger, who picked a starting point so far back that three old cycles — 28 years of weekdays, 19 years of moon phases, 15 years of Roman tax indictions — had all reset together on the same day. That combined cycle runs 7980 years (15 × 19 × 28), so the count runs from Monday, 1 January 4713 BC on the Julian calendar, and no date in recorded history has a negative day number. Astronomers still use it because subtracting two JDNs gives "days between" with nothing to memorize: no leap rules, no month lengths, no calendar reforms. The count changes at noon, not midnight, so that a night's observation never straddles two dates. (The name honors Scaliger's father Julius, not Julius Caesar.) This is the same day count the date calculator uses under the hood.

The Julian calendar: honest, and slightly wrong

Julius Caesar's reform of 46 BC (after a 445-day "year of confusion" that realigned the seasons) made every fourth year 366 days. That overcounts by about 11 minutes a year — the year is 365.2422 days, not 365.25 — and eleven minutes adds up: one full day of drift every 128 years or so. By the 1500s the spring equinox had slid ten days off its appointed date, and the church's Easter calculations were a mess. Pope Gregory XIII's fix in 1582 was characteristically blunt: skip ten days. Thursday, 4 October 1582 on the Julian calendar was followed by Friday, 15 October on the new Gregorian one. The Gregorian rule (leap years every four years except centuries not divisible by 400) is itself a contraption — accurate to one day in about 3,200 years — and the Julian calendar never adopted it. It still runs 13 days behind the Gregorian today, which is why Christmas falls on 7 January in countries that keep it, and the gap widens to 14 days in 2100, when the Julian leap day has no Gregorian twin to pair with.

ISO weeks: the calendar inside the calendar

Weeks are the one unit no calendar reconciles: seven days divides neither the month (28–31 days) nor the year (365 or 366). ISO week dates solve this by ignoring months altogether. Every week has exactly seven days, Monday first; week 1 is the one containing the year's first Thursday (equivalently, the one with 4 January in it); and a year has 52 weeks — unless it starts on a Thursday, or is a leap year starting on a Wednesday, in which case 53. The result reads like a serial number — 2026-W36-3 means the Wednesday of week 36 — because that is essentially what it is: factories plan by it, financial quarters are compared by it, and 2026 happens to be one of the 53-week years. The cost is that a week's year number can differ from the calendar year on its edges: 1 January can belong to week 52 or 53 of the previous year, and 31 December to week 1 of the next.

The Maya: five gears

The Long Count is a day count like the Julian Day Number — but displayed as an odometer with five gears, mostly base 20. From the right: kin (days), then uinal (18 per tun), tun (20 per katun), katun (20 per baktun). One odd 18-toothed gear keeps the tun at a tidy 360 days — 18 × 20 — which is close to a solar year and clearly not an accident. A baktun is 144,000 days, and the whole five-gear display resets after thirteen of them: 1,872,000 days, about 5,125 years. Day zero of the current cycle corresponds to 11 August 3114 BC, and 21 December 2012 was simply the odometer reading 13.0.0.0.0 — a round number, much advertised, and followed the next day by 13.0.0.0.1. The next similarly round reading, 14.0.0.0.0, falls on 26 March 2407. Matching Long Counts to our calendar requires a fixed offset from Julian Day Numbers; the standard one (called the GMT correlation) is exactly the 584,283 this page uses.

The French Republican: the calendar that needed an astronomer

Revolutionary France threw out the saints, the Sundays and the emperors in one go. The new calendar (adopted in October 1793, backdated to its epoch) had twelve months of thirty days, each cut into three ten-day décades — with the tenth day, décadi, as the day of rest — plus five or six spare days at year's end for national holidays. The months were renamed for weather and farm work by the actor Fabre d'Églantine: Vendémiaire (grape harvest) through Fructidor (fruit). A British wit, less charmed, translated them as Wheezy, Sneezy, Freezy, Slippy, Drippy, Nippy, Showery, Flowery, Bowery, Hoppy, Croppy and Poppy. Years were numbered from the founding of the Republic: 22 September 1792 is 1 Vendémiaire, An I.

Here is the contraption part. The decree says the year begins on the day the true autumn equinox falls at the Paris Observatory and it defines a four-year cycle with a leap year (sextile) at the end. These two rules contradict each other — equinoxes do not obey four-year schedules. In practice the astronomers won: years III, VII and XI were observed as six-day years, XV and XX were planned five years apart, and nobody had to resolve the clash because Napoleon abolished the calendar on 1 January 1806 (the Paris Commune briefly revived it for 18 days in 1871 — dates in An LXXIX). Every converter since inherits the mess: most use a tidy arithmetic stand-in for the equinox rule, and they can disagree by a day or two. This page computes what the law actually said — the September equinox as seen from the Paris Observatory. That means undoing the clock drift of the Earth (ΔT), applying the equation of time, and shifting from Greenwich to the observatory's longitude, 2°20′15″ east — a correction of 9 minutes 21 seconds. The whole chain lives in this page's source, runs entirely in your browser, and is why An 234 — the year you are reading this — is a six-day year: the 2026 equinox falls on 23 September in Paris, so An 234 spans 366 days and ends on 22 September 2026, Fête de la Révolution, with An 235 beginning the next morning.

What this page doesn't do

These are calendar dates, not instants: nothing here depends on your time zone, and "today" means your device's own date. Dates before 22 September 1792 are refused — the French Republican calendar did not exist, not even retroactively. The equinox computation above is dependable through the year 2200; past that, the Earth's slowing rotation (ΔT) is extrapolated, and a few seconds of uncertainty is all it would take to move an equinox across a midnight. And the five counters on this page are not rivals — every one of them is counting the exact same day, five different ways.