The modern Solar Hijri calendar used to determine Nowruz was adopted in 1925.
Iran legally adopted the modern calendar on 31 March 1925, during the early Pahlavi period. The reform established a solar year beginning with the first day of spring and restored the traditional Persian names for the months, including Farvardin, Ordibehesht, and Khordad. It also fixed the month lengths: the first six months have 31 days, the next five have 30, and the final month has 29 or 30 in a leap year.
The calendar is “Hijri” because its year numbering begins with the Hijra, Muhammad’s migration from Mecca to Medina in 622 CE. It is “Solar” because its year follows the seasonal cycle and begins at the vernal equinox rather than following the lunar phases of the Islamic calendar.
A common mix-up is confusing the 1925 legal adoption with later calendar changes. In 1976, Iran briefly introduced an Imperial calendar with a different era numbering, but the Solar Hijri system was restored in 1978. Nowruz remains the first day of Farvardin.