The Hijri calendar (Arabic: ٱلتَّقْوِيم ٱلْهِجْرِيّ, *al-taqwīm al-hijrī*), commonly known as the Islamic calendar, is more than just a date-tracking system; it is a profound cosmological and religious framework. Unlike the widely-used solar Gregorian calendar, the Hijri is a purely lunar calendar, establishing a unique and continuously shifting relationship with the Earth's seasons.

This calendar is the foundation for all major Islamic observances, dictating the dates for Ramadan, Eid al-Fitr, Eid al-Adha, and the annual Hajj pilgrimage.

Part I: Historical and Religious Foundation

The Hijri calendar is intrinsically linked to the founding event of the Islamic faith, giving it an unparalleled spiritual significance.

1. The Epoch: The Significance of the Hijrah

The era of the Islamic calendar is known as the Hijri Era, denoted by AH (*Anno Hegirae*, or 'In the Year of the Hijra').

• The Starting Point: The calendar begins not with the Prophet Muhammad's birth or the date of the revelation, but with the Hijrah (the migration). This was the journey of the Prophet and his early followers from Mecca to Medina (then Yathrib) in 622 CE.
• The Religious Meaning: The selection of the Hijrah signifies a fundamental shift: it marks the establishment of the first sovereign Muslim community (*Ummah*) and the transition from a period of persecution to a period of organization and growth. It is a spiritual pivot point, representing the start of the Islamic society itself.
• Official Adoption: The calendar was formally adopted and established as the official dating system for the Islamic State approximately 17 years after the actual Hijrah, during the reign of the second Caliph, Umar ibn Al-Khattab (638 CE). After deliberation with his advisors, the year of the migration was chosen as Year 1, and the month of Muharram was selected as the first month of the year, continuing the pre-Islamic month order.

2. The Great Reform: Prohibition of *Nasī’*

Before Islam, the Arabs of the Hijaz region used a Lunisolar Calendar that involved a practice called ***Nasī’*** (literally, "postponement" or "intercalation").

• The Pre-Islamic System: The calendar was mostly lunar, but a month was occasionally added (intercalated) to realign the pilgrimage season (**Hajj**) with the solar year. This ensured the Hajj and associated fairs always fell at a convenient time for trade and travel.
• The Islamic Correction: The Quran explicitly forbade the practice of *Nasī’*. This prohibition, often understood to have been announced during the Prophet's Farewell Pilgrimage, made the calendar purely lunar. This was a profound religious reform that completely disconnected the Islamic calendar from the solar seasons and any commercial manipulation.

Part II: The Scientific and Astronomical Basis

The greatest scientific distinction of the Hijri calendar is its adherence to the observed lunar cycle, making it fundamentally different from all solar systems.

1. A Purely Lunar System

The Islamic year is determined solely by the lunar cycle (the synodic month), not the Earth's orbit around the Sun.

• Year Length: The average lunar year consists of 12 lunar months, resulting in approximately 354.367 days. This makes the Hijri year approximately 10 to 11 days shorter than the 365.24-day Gregorian (solar) year.
• The Seasonal Shift (The Wisdom of *Taqwīm*): Because the Islamic calendar is disconnected from the seasons, its months regress through the entire solar year over a period of roughly 33 years. This means Muslims observe holy days like Ramadan and Hajj in all seasons—sometimes in the long, hot days of summer, and sometimes in the short, cool days of winter—ensuring all people experience the rituals under different climatic conditions.

2. Month Structure and Visibility (The Observational Standard)

The start of a Hijri month is determined by the crescent Moon sighting (*hilāl*).

3. The Tabular Calendar (The Calculation Standard)

For civil, non-religious purposes (like publishing a consistent calendar or converting historical dates), an arithmetical approximation is often used: the Tabular Islamic Calendar.

• The Cycle: This calendar uses a 30-year cycle where 11 of the 30 years are designated as "leap years."
• Leap Year Rule: In the leap years of the 30-year cycle, the last month, Dhū al-Hijjah, is given 30 days instead of the usual 29. This maintains the long-term average of the lunar year. This tabular method allows for consistent, predictable calculation without reliance on physical observation.

Part III: Key Differences and Conversion

Approximate Conversion

Because the two calendars are of different lengths and shift annually, no simple formula provides a precise, day-for-day conversion. However, for a quick estimate, the following formulas are often used by historians and statisticians:

AH ≈ (CE - 622) * 33/32

CE ≈ AH * 32/33 + 622

For example, 2025 CE corresponds roughly to the Hijri year 1446 AH through 1447 AH. Because the Hijri New Year (1 Muharram) shifts, a single Gregorian year always overlaps with two, and sometimes three, Hijri years.