Hijri Calendar Converter – Tutorial

Hijri Calendar Converter — Detailed Guide (Gregorian → Hijri)

This page explains step by step how a Gregorian (Miladi) date is converted to a Hijri date. The calculator's approach: convert the Gregorian date to a common "day count" system, then derive the Hijri year/month/day from that count.

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1) What is the Hijri Calendar?

The Hijri calendar is a lunar calendar based on the phases of the Moon. The Gregorian calendar is a solar calendar based on the Earth's orbit around the Sun.

• Gregorian year ≈ 365.2422 days (solar year)
• Hijri year ≈ 354 or 355 days (lunar year)
• So the Hijri year falls about 10–11 days earlier each Gregorian year.

Important: Hijri months can be observational (based on crescent moon sighting), so results may differ by 1 day between countries. This calculator uses the astronomical/calculated method.

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2) Why Can't We Convert Directly? (The core idea)

Gregorian months have 28–31 days. Hijri months have 29 or 30 days. Since the two calendars follow different logic, converting "2025-12-28 = which Hijri day" requires a common bridge between the two systems.

That bridge is: Julian Day Number.

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3) What is the Julian Day Number?

The Julian Day is a system that converts any date into a sequential day count. Instead of dealing with complex month/year structures, it answers: "How many days have passed since the beginning?"

• Each day = +1
• Calculating differences between dates becomes trivial
• Serves as a common language for Gregorian ↔ Hijri conversions

The calculator first produces a Julian Day from the Gregorian date:

Gregorian Date → Julian Day → Hijri Date

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4) How Does This Calculator Work?

4.1 Parses the Gregorian date

• day, month, year

Then checks if the year is after 1583. The Gregorian calendar was introduced around 1582, so algorithms are reliable only after that point.

4.2 Calculates the Julian Day

• Sums the days in the year (365.25 approximation)
• Adds the month contribution (30.6001 approximation)
• Applies the Gregorian correction (century correction)
• Result: julianDay

4.3 Subtracts the Hijri epoch

A fixed "start day" constant for the Hijri calendar:

iEpoch = 1948439.5

This references the Julian Day of the Hijri calendar's start (the Hijra). Then:

iDay = julianDay - iEpoch

"How many days have passed since the Hijri beginning to this Gregorian date?"

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5) How is the Hijri Year Found? (30-year cycle)

The Hijri calendar has a leap year system based on a 30-year cycle. Total days in 30 Hijri years:

10,631 days

• cyc = iDay / 10631 — how many complete 30-year cycles have passed
• rem — remaining days within the current cycle

Then the remaining days are divided by the approximate Hijri year length (354.36667):

iYear ≈ rem / 354.36667

This is a calculated approximation. Real-world institutions (like Diyanet) may also use observation/astronomy tables, which can result in 1-day differences.

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6) How Are the Hijri Month and Day Found?

6.1 Hijri months alternate between 29 and 30 days

• 30, 29, 30, 29, ... alternating
• The last month (Dhu al-Hijjah) may have 30 days in leap years

6.2 Leap year check

Some Hijri years have 355 days. The check:

(iYear * 11 + 14) % 30 < 11 → leap year

• If leap year: monthLengths[11] = 30 (Dhu al-Hijjah = 30 days)

6.3 Finding month and day by subtracting

"rem" represents the day within the year. Month lengths are subtracted one by one:

• If rem ≤ first month length: we're in month 1
• Otherwise: rem -= that month's length, move to next month

Result: hMonth (which month) and hDay (day of month).

6.4 Why is there a "hDay = 0" correction?

At exact boundary cases, the remaining days can appear as 0. Since there's no "day 0" in a calendar, the code shifts back to the previous month's last day.

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7) Hijri Month Names

• 1 Muharram
• 2 Safar
• 3 Rabi al-Awwal
• 4 Rabi al-Thani
• 5 Jumada al-Awwal
• 6 Jumada al-Thani
• 7 Rajab
• 8 Sha'ban
• 9 Ramadan
• 10 Shawwal
• 11 Dhu al-Qi'dah
• 12 Dhu al-Hijjah

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8) Special Day Detection (Ramadan / Eid / Islamic New Year)

• Month of Ramadan: hMonth = 9
• Start of Eid al-Fitr: hMonth = 10 and hDay = 1
• Start of Eid al-Adha: hMonth = 12 and hDay = 10
• Islamic New Year: hMonth = 1 and hDay = 1

Note: Official holiday declarations may differ by 1 day between countries/institutions. This tool uses the calculated method and is for informational purposes only.

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9) What Does the Result Table Show?

The table breaks the date into its components for easy verification:

• Day: hDay
• Month: Hijri month name
• Year: finalHYear
• Gregorian Reference: selected date

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10) Why Can Results Differ by 1 Day?

• Observational (crescent): month starts when the crescent moon is sighted
• Calculated (astronomical): month starts based on pre-calculated tables

Institutions like Diyanet and Umm al-Qura use specific astronomical criteria. Different criteria can shift some dates by 1 day.

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11) How to Use This Calculator Accurately

1. Select a Gregorian date (e.g., 2025-12-28)
2. Read the Hijri date result
3. Check for special day labels (Ramadan, Eid, etc.)
4. For official religious dates, always refer to your local institution's calendar (e.g., Diyanet)

Warning: This tool is for informational purposes only. For official announcements and religious observances, always use the calendar published by the relevant institution.