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Iran Uses the Most Accurate Calendar on Earth, and It Has No Formula

The Gregorian calendar is a good calendar. It stays aligned with the sun by means of a rule — divisible by four, except centuries, unless divisible by four hundred — and that rule is accurate enough that it drifts by one day roughly every 3,236 years.

The Solar Hijri calendar, official in Iran since 1925, does something different. It does not use a rule at all.

Each Iranian year begins at the exact instant of the vernal equinox, as astronomically determined for the meridian at 52.5° east, about 400 kilometres from Tehran. That moment is Nowruz, and it lands on 20 or 21 March. Whether the final month, Esfand, gets 29 days or 30 is decided by nothing more than how many days actually elapsed between one equinox and the next.

There is no arithmetic to memorise, because there is no arithmetic. The sun decides, every year, individually.

The result is a calendar that essentially cannot drift. The best mathematical approximation anyone has proposed for it — a scheme built on 2,820-year periods subdivided into cycles of 29, 33 and 37 years — takes about 110,000 years to accumulate a single day of error.

That is roughly thirty-four times more accurate than the calendar most of the world uses.

The months are named for the seasons and the year is split cleanly: the first six months have 31 days, the next five have 30, and Esfand takes whatever is left.

The clock: Iran does the same thing with its time zone that it does with its calendar. It refuses to round.

Most countries accept a whole-hour offset and live with being twenty, forty, sixty minutes out of step with their own sun. Iran runs on UTC+3:30 — one of the world's few half-hour zones — chosen so that the national meridian at 52.5° east lands almost exactly on a clean offset from Greenwich.

Tehran sits at 51.4° east. Its clock is within a few minutes of its own solar noon.

On a list containing Madrid at an hour out, Istanbul at sixty-four minutes, Buenos Aires at fifty-four and Lagos at forty-six, Tehran is the city that simply declined to be wrong — and pays for it by permanently confusing everybody else's scheduling software.

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