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. 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
| Persian year | Leap? | Esfand length |
|---|---|---|
| 1399 | Yes | 30 days |
| 1400 | No | 29 days |
| 1403 | Yes | 30 days |
| 1404 | No | 29 days |
| 1408 | Yes | 30 days |
| 1412 | Yes | 30 days |
| 1416 | Yes | 30 days |
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.
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