How the Solar Hijri calendar works
The first six months, Farvardin to Shahrivar, have 31 days each; the next five, Mehr to Bahman, have 30; and Esfand has 29 days, or 30 in a leap year. Years are counted from the Hijra in 622 CE, like the lunar Hijri calendar, but because they are solar years the two counts drift apart: 1405 Solar Hijri runs alongside 1447 and 1448 lunar Hijri. Afghanistan uses the same calendar with the zodiac month names Hamal, Sawr, Jawza and so on, which the month table shows in Dari.
Nowruz and the equinox
The year begins at the March equinox, the moment the sun crosses the celestial equator heading north. By the rule used in Iran, if the equinox comes before noon Iran time, that day is Nowruz, 1 Farvardin; if it comes after noon, Nowruz is the next day. That is why Nowruz is on 20 March in some years and 21 March in others. Nowruz is a spring new year marked by many communities and cultures across Iran, Afghanistan, Central Asia, the Caucasus and the Kurdish regions, each with its own traditions.
Leap years
Because the year is tied to the real equinox, leap years are not on a fixed four-year pattern: usually every fourth year, but sometimes after five. The browser's Persian calendar follows an arithmetic rule that matches the astronomical calendar for the years this tool covers; testing found no year from 1900 to 2100 where the noon rule above gives a different Nowruz. When the equinox is within an hour of noon in Iran, the tool still shows a reminder to check the official calendar.
The equinox moment
The equinox time is calculated with the method in Jean Meeus's Astronomical Algorithms, chapter 27, with a correction for the difference between atomic and Earth-rotation time. For every year from 1980 to 2080 it matched the times published by the US Naval Observatory to within 2 minutes, and by about 20 seconds on average. It is shown in UTC, in Iran Standard Time (UTC+3:30) and in your device's time zone, which only changes the clock time shown, never the calendar date of Nowruz.
Limits
Dates before March 1900 and after March 2100 are not offered. The official calendar in Iran is published by the calendar centre of the University of Tehran's Institute of Geophysics; we could not open that site from our test environment, so the dates here rest on the browser's Persian calendar and the checks described above. For official, community or religious observances, follow the calendar announced in your country or community. This tool is general help, not religious advice.