Calendar Guide
How each calendar arose, what changes it went through from its first day to the present, what its rules are, and where it stands in the classification of calendar scholars. Each calendar has a detailed page with its sources.
How calendars are classified
Every calendar rests on three independent natural units: the day (the succession of night and daytime), the month (the moon’s cycle, about 29.53 days), and the year (the cycle of the seasons, about 365.2422 days). Because they do not divide into each other in whole numbers, each calendar chooses which of them to follow, and is classified accordingly on two axes:
By the unit it follows
- Solar: follows the year alone; its months are conventional divisions unrelated to the moon (Gregorian, Julian, Solar Hijri).
- Lunar: follows the lunar month alone, so its months drift through the seasons (Lunar Hijri).
- Lunisolar: its months are lunar, and a leap month is added to keep it in its seasons (Chinese, Hebrew, The Creator's Calendar).
By the method of calculation
- Observational: relies on sighting, such as the sighting of the crescent.
- Arithmetical: fixed rules and averages, computable in advance for any date (Julian, Gregorian, Hebrew).
- Astronomical: the true positions of the sun and moon at computed moments (Chinese since 1645, Solar Hijri, The Creator's Calendar).
Comparison table
| Calendar | Type | Calculation | Month | Year | Intercalation | Start of day |
|---|---|---|---|---|---|---|
| The Creator's Calendar | Lunisolar | Astronomical | 29 or 30 days from conjunction (Madinah reference) | 12 or 13 months (354–355 / about 384 days) | Al-Nasi: a year with 13 new moons | Dawn |
| Chinese | Lunisolar | Astronomical (since 1645) | 29 or 30 days from the new moon | 12 or 13 months (353–355 / 383–385 days) | A month without a principal term in a 13-month suì | Midnight (UTC+8) |
| Gregorian (Julian and Gregorian) | Solar | Arithmetical | Fixed divisions: 28–31 days | 365 or 366 days | Every 4 years; the Gregorian drops 3 in every 400 | Midnight |
| Hebrew | Lunisolar | Arithmetical (since the 4th–10th centuries CE) | 29 or 30 days from the mean “molad” | 12 or 13 months (353–355 / 383–385 days) | 7 years in every 19 (Adar I) | Sunset |
| Solar Hijri | Solar | Astronomical (the March equinox) | Fixed divisions: 31, then 30, then 29/30 days | 365 or 366 days | By observation: about 8 in every 33 years | Midnight (UTC+3:30) |
| Lunar Hijri | Lunar | Observational (with arithmetical and tabular forms) | 29 or 30 days from the crescent | 12 months (354 or 355 days) | None (tabular: 11 days in every 30 years) | Sunset |
The calendars
The Creator's Calendar
True lunar months anchored in their seasons by Al-Nasi, a day that begins at dawn, and Madinah as its reference: its rules, its linguistic evidence from Lisan al-Arab, and the evidence of events.
The Chinese calendar
From the Shang oracle-bone inscriptions to the 2017 national standard: the solar terms, the leap month, the sexagenary cycle, and more than two thousand years of reforms.
The Gregorian calendar: Julian and Gregorian
From the chaos of the Roman year to the reforms of Julius Caesar and Gregory XIII: the leap rule, the ten dropped days, the spread of the Gregorian country by country, and the month names used by the Arabs.
The Hebrew calendar
From sighting the crescent before the Sanhedrin to fixed arithmetical rules: the molad, the nineteen-year cycle, the postponements of the new year, the Saadia–Ben Meir dispute, and al-Khwarizmi’s treatise.
The Solar Hijri calendar
From the Zoroastrian calendar to the Jalali era and the 1925 law: Nowruz on the day of the equinox, the noon rule at 52.5° E, astronomical leap years, and the month names and their origins.
The Lunar Hijri calendar
The months and their names, the Hijri era, sighting, Umm al-Qura, astronomical criteria and the tabular calendar, a comparison of month starts, and the question of Al-Nasi between the prevailing view and the thesis of The Creator's Calendar.