On March 20, Iranians rang in the year 1404 of the Persian calendar, marking the astronomical moment of the spring equinox. Around the world, an estimated 300 million people will wish one another a happy new year—Norouz mobarak in Persian—not just in Iran but also in Afghanistan, Kazakhstan, and among Kurdish communities in Turkey and Iraq.

The Persian calendar, which has Zoroastrian roots, begins with the Hijrah—the Prophet Muhammad’s migration from Mecca to Medina—in 622 A.D. Officially adopted in Iran in 1925, this calendar alternates between common and leap years.

While the Gregorian calendar is the most widely used system today—especially in Western countries—dozens of other calendars remain in use across the globe, shaped by religious traditions and regional histories.

Solar calendars

Like the Persian calendar, the Julian and Gregorian calendars are based on the solar cycle. The Julian calendar, introduced by Julius Caesar in the first century B.C., aimed to match the Earth’s orbit around the sun with a 365-day year. It added a leap day every four years to account for the actual solar year of roughly 365.2422 days.

However, the Julian calendar’s slight inaccuracy led to a cumulative shift of about 10 days by the end of the 16th century, disrupting the timing of religious holidays like Easter. To fix this, Pope Gregory XIII eliminated 10 days from the calendar in 1582, jumping directly from Oct. 4 to Oct. 15—thus creating the Gregorian calendar we use today.

Still, some Orthodox churches, such as those in Russia and Jerusalem, continue to follow the Julian calendar. In May 2023, the Council of Bishops of the Orthodox Church of Ukraine voted to adopt a revised Gregorian liturgical calendar—signaling further separation from the Moscow Patriarchate, which it split from in 2018.

Interestingly, these calendars have no official “year zero”; they shift directly from 1 B.C. to A.D. 1, based on the estimated birth year of Christ, as calculated by the 6th-century monk Dionysius Exiguus, according to retired educator Jacques Gispert in an interview with France Culture.

Other solar calendars include the Coptic calendar—dating back to the rise of Emperor Diocletian in 284 A.D.—which influenced the Ethiopian calendar. The Tamil calendar, now in its year 5125, also follows solar rhythms.

Lunar calendars

The Islamic, or Hijri, calendar also begins with the Hijra. Based on lunar cycles, its 12 months contain either 29 or 30 days, producing years of 354 or 355 days. The start of each month is determined by the naked-eye sighting of the new moon, which can vary by country.

Because of this variability, the Hijri calendar is not used for official civil purposes, though it remains vital for determining the timing of Muslim holidays—explaining why Ramadan shifts about 11 days earlier each year. According to this calendar, the year 2025 in the Gregorian system corresponds to 1446.

Lunisolar calendars

Many calendars blend both solar and lunar cycles. The Hebrew calendar, for instance, aligns its months with the lunar cycle while maintaining the 12-month solar year.

Each month contains 29 days, and to realign with the solar year, an extra “intercalary” month is added seven times every 19 years. The calendar begins in 3761 B.C., the traditional Jewish date of creation. By that count, the year 2025 corresponds to 5785.

The Chinese calendar is another lunisolar system. It begins each year with the new moon and adds a 13th month when necessary to keep pace with the sun—explaining why the Chinese New Year can fall anywhere between Jan. 21 and Feb. 19.

The Buddhist calendar, used in Southeast Asia, begins with the death of Buddha, estimated around 543 B.C. In that system, the year 2025 would be 2566.