Persian (Jalali) leap years, explained

The Gregorian leap-year rule is famously simple: a year is a leap year if it is divisible by 4, except centuries that are not divisible by 400. The Persian (Jalali / Solar Hijri) calendar also adds an extra day every few years, but its rule is different, and getting it right matters whenever you convert a date near the end of the Persian year.

The extra day lands on 30 Esfand

In an ordinary Persian year the final month, Esfand, has 29 days. In a leap year Esfand gains a 30th day. That is the whole visible effect of a Persian leap year: the date 30 Esfand simply does not exist in an ordinary year.

This is the Persian counterpart to 29 February. So a date like 30 Esfand 1403 is valid only because 1403 is a leap year (it corresponds to 20 March 2025), whereas “30 Esfand 1404” is not a real date at all. A good converter will reject it rather than silently roll it into Farvardin.

The rule: a 33-year cycle, not “divisible by 4”

Because the Persian calendar keeps 1 Farvardin pinned to the spring equinox (see why Nowruz drifts on the Gregorian calendar), its leap years cannot follow the tidy Gregorian formula. Instead, the widely used arithmetic approximation distributes 8 leap years across a 33-year cycle.

In practice that means leap years usually come every 4 years, with an occasional 5-year gap to keep the calendar in step with the seasons over the long run (source: Encyclopædia Britannica, “Iranian calendar”). For example, across the years 1403 to 1408 the leap years are 1403 and 1408 — a five-year gap, not the four you’d assume. The converter uses the well-established jalaali-js implementation of this cycle, which is accurate across the range of years people actually convert.

A quick way to picture it: count any 33 consecutive Persian years and you will find exactly 8 of them are leap years. Most of the gaps between them are 4 years; one is 5.

Recent and upcoming Persian leap years

Between 1403 and 1408 there is a 5-year gap, the irregular step that the simple “every 4 years” intuition misses. If you assume Persian leap years always fall four years apart, you will eventually place 30 Esfand in the wrong year.

Practical takeaways

• The extra day is always 30 Esfand, never inserted mid-year.
• Leap years are roughly every 4 years, but not exactly. The 33-year cycle includes a 5-year gap.
• When in doubt, convert the specific date instead of applying the Gregorian rule to a Persian year.

You can verify any of this yourself: open the Jalali ⇄ Gregorian converter and try entering 30 Esfand in a leap year versus an ordinary one. The leap day’s effect on Nowruz’s Gregorian date is covered in why Nowruz isn’t always on the same Gregorian day, and counting spans that cross a leap day is covered in days between dates.