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Iranians use three calendars in daily life:

- Gregorian calendar for practical matters

- Islamic calendar for religious matters

- Persian calendar for matters of hubris

The modern Persian calendar is a solar calendar (sharing the same year 0 as the Islamic calendar), and the first day of the year is always synchronized with the Spring equinox.

Edit: Although the Persian and Islamic calendars share the same initial year, the (lunar) Islamic calendar is 11 days shorter. Over time, this has resulted in the current year being "decades" apart between the two.

https://en.wikipedia.org/wiki/Iranian_calendars#Modern_calen...