Gregorian vs Solar Hijri: two calendars on one timeline

Two calendars, one timeline. Every day that has ever happened gets a Gregorian label and a Solar Hijri label, and both are correct, just different coordinate systems laid over the same stretch of time. The confusing part is that they don’t agree on where the year starts or what to call it.

Convert any date between the two here:

The year offset isn’t a fixed number

You’ll often hear “subtract 621” to go from Gregorian to Solar Hijri. That’s roughly right, but the exact offset is either 621 or 622 depending on the time of year, because the two calendars start their years months apart.

The Solar Hijri year begins at the March equinox. So from late March through December, a Gregorian year overlaps the Solar Hijri year that started that March (offset 621). But in January through late March, you’re still inside the Solar Hijri year that began the previous March, so the offset is 622.

Here’s how 1 January maps across recent years. Note it always lands in Dey, the tenth Persian month, not at the start of the Persian year:

Where each Solar Hijri year sits on the Gregorian calendar

Flip it around and the overlap is clearer. A Solar Hijri year straddles two Gregorian years, running from one March to the next:

That March-to-March span is why a single Solar Hijri year touches two Gregorian years, and why the new-year dates never quite line up. The start date wobbles between 20 and 21 March too, for reasons I covered in why Nowruz drifts on the Gregorian calendar.

Same length, different leap days

Both calendars run about 365.24 days a year, so over time they stay locked together, with neither drifting relative to the other. What differs is how each one absorbs the extra quarter-day. Gregorian uses the 4/100/400 rule and adds 29 February. Solar Hijri uses a 33-year cycle and adds 30 Esfand. The mechanisms differ, but the goal is the same: stay glued to the Sun.

Because the leap days fall in different months, a given Gregorian leap year and the overlapping Solar Hijri year aren’t necessarily both leap. That’s the kind of edge case the converter handles for you, and if you’re measuring spans across the two systems, the date difference tool counts the real days regardless of calendar.