Islamic Date Calculator
Calculate days between two Hijri dates or convert Hijri date ranges to Gregorian. Enter values for instant results with step-by-step formulas.
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This formula converts a Hijri date to a Julian Day Number (JD) using the tabular Islamic calendar. Y is the Hijri year, M is the month, and D is the day. The Julian Day Number can then be converted to Gregorian or used to compute date differences.
Last reviewed: December 2025
Worked Examples
Example 1: Days Between Two Hijri Dates
Example 2: Hijri to Gregorian Conversion
Background & Theory
The Islamic Date Calculator applies the following established principles and formulas. Date and time calculations underpin a vast range of applications from financial settlement to scheduling and age verification. The complexity arises because civil timekeeping uses irregular units: months have 28, 29, 30, or 31 days; years have 365 or 366 days; hours, minutes, and seconds use base-60 arithmetic; and time zones introduce offsets ranging from -12:00 to +14:00 relative to UTC. The Gregorian calendar's leap year rule is a compound condition: a year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. Thus 1900 was not a leap year but 2000 was. This rule keeps the calendar synchronized with the solar year to within about 26 seconds per year. For algorithmic date calculations, the Julian Day Number provides a continuous integer count of days since January 1, 4713 BCE, eliminating the irregularity of calendar months and making interval arithmetic straightforward. The Unix epoch, by contrast, counts seconds since 00:00:00 UTC on January 1, 1970, and is the basis of POSIX time used in most computing systems. ISO 8601 standardizes date and time representation as YYYY-MM-DD and combined datetime as YYYY-MM-DDTHH:MM:SS±HH:MM, ensuring unambiguous machine-readable interchange across locales that would otherwise differ in day/month/year ordering. Business day calculation requires excluding weekends and, optionally, a jurisdiction-specific list of public holidays. Duration calculations expressed in years, months, and days must account for the variable length of months, making them non-commutative: the interval from January 31 to February 28 is different from the interval from February 28 to March 31. Age calculation algorithms must handle the edge case of birthdays on February 29 and ensure that a person born on December 31 is not counted as one year older on January 1 of the following year until the clock passes midnight. Zeller's Congruence provides a closed-form formula to determine the day of the week for any Gregorian or Julian calendar date using only integer arithmetic.
History
The history behind the Islamic Date Calculator traces back through the following developments. The need to track time and predict astronomical events gave rise to calendrical systems independently across many civilizations. The Babylonians, around 2000 BCE, developed a lunisolar calendar with 12 months of alternating 29 and 30 days, inserting an intercalary month periodically to keep pace with the solar year. They also divided the day into 24 hours and the hour into 60 minutes, a sexagesimal convention that persists in every modern clock. The Egyptian civil calendar used 12 months of exactly 30 days plus five epagomenal days, totaling 365 days. Though simple for administrative purposes, it drifted against the solar year by one day every four years. Julius Caesar, advised by the Egyptian astronomer Sosigenes, reformed the Roman calendar in 45 BCE. The Julian calendar introduced a 365-day year with a leap day every four years, a system that served Europe for over sixteen centuries. By the 16th century, the accumulated error of the Julian calendar had shifted the spring equinox ten days from its ecclesiastically mandated date, disrupting the calculation of Easter. Pope Gregory XIII commissioned the calendar reform that bears his name, and the Gregorian calendar was introduced in Catholic countries in October 1582. The transition required skipping ten days: October 4 was followed by October 15. Protestant and Orthodox countries adopted the reform slowly; Britain and its colonies switched in 1752, Russia not until 1918, and Greece in 1923. The expansion of railways in the 1840s created an urgent practical problem: each city operated on its own local solar time, making train timetables impossible to coordinate. British railways adopted Greenwich Mean Time as a standard in 1847. The International Meridian Conference of 1884 in Washington formalized the prime meridian at Greenwich and established the global framework of 24 time zones. Daylight saving time was first adopted nationally during World War I to reduce coal consumption. The development of atomic clocks after World War II led to the definition of Coordinated Universal Time (UTC) in 1960, accurate to nanoseconds. The Y2K problem of 1999-2000 demonstrated that two-digit year storage in legacy systems could cause widespread failures, prompting a global remediation effort costing an estimated 300 to 600 billion dollars.
Key Features
- Calculate the exact difference between any two dates expressed in days, weeks, months, and years simultaneously, accounting for leap years and varying month lengths.
- Add or subtract any combination of years, months, weeks, and days from a starting date to determine a precise future or past date, with results shown in a full calendar format.
- Compute a person's exact age from their birthdate in years, months, and days as of today or any specified reference date, suitable for legal, medical, and personal use.
- Count business days between two dates by excluding weekends and optionally filtering out public holidays from a configurable set of regional holiday calendars.
- Display a live countdown to any target date and time showing the remaining years, months, days, hours, minutes, and seconds, updating in real time.
- Convert a specific date and time between any two IANA time zones, correctly handling daylight saving time transitions and historical offset changes.
- Determine the day of the week for any historical or future date using the proleptic Gregorian calendar, supporting dates ranging from antiquity through far-future years.
- Format a calculated duration in ISO 8601 interval notation as well as plain human-readable text such as '2 years, 4 months, and 11 days' for use in documentation and APIs.
Frequently Asked Questions
Formula
JD = floor((11Y+3)/30) + 354Y + 30M - floor((M-1)/2) + D + 1948440 - 385
This formula converts a Hijri date to a Julian Day Number (JD) using the tabular Islamic calendar. Y is the Hijri year, M is the month, and D is the day. The Julian Day Number can then be converted to Gregorian or used to compute date differences.
Worked Examples
Example 1: Days Between Two Hijri Dates
Problem: Calculate the days between 1 Muharram 1446 AH and 15 Jumada al-Thani 1446 AH.
Solution: 1 Muharram 1446 = JD 2460493 (approx July 7, 2024)\n15 Jumada al-Thani 1446 = JD 2460657 (approx Dec 18, 2024)\nMonths: Muharram(30) + Safar(29) + Rabi I(30) + Rabi II(29) + Jumada I(30) + 14 days of Jumada II\nDays = 30 + 29 + 30 + 29 + 30 + 14 = 162 days\nWeeks = 23 weeks and 1 day
Result: 162 days | 23 weeks 1 day | Approx 5.5 months
Example 2: Hijri to Gregorian Conversion
Problem: Convert 1 Ramadan 1447 AH to the Gregorian calendar.
Solution: Using the tabular Hijri-to-JD conversion:\nJD = floor((11 x 1447 + 3) / 30) + 354 x 1447 + 30 x 9 - floor(8/2) + 1 + 1948440 - 385\nConvert JD to Gregorian date using standard algorithm.\n1 Ramadan 1447 AH falls in February 2026.
Result: 1 Ramadan 1447 AH = approximately February 2026 CE
Frequently Asked Questions
How does the Islamic (Hijri) calendar work?
The Islamic calendar, also known as the Hijri calendar, is a purely lunar calendar consisting of 12 months that alternate between 29 and 30 days. A standard Hijri year contains 354 days, which is about 11 days shorter than the Gregorian solar year. This means Islamic dates shift earlier relative to the Gregorian calendar by approximately 11 days each year. The calendar begins from the year of the Hijra, Prophet Muhammad's migration from Makkah to Madinah in 622 CE. Odd-numbered months have 30 days and even-numbered months have 29 days, except in leap years when the 12th month Dhul Hijjah has 30 days instead of 29.
How accurate is the tabular Islamic calendar for conversions?
The tabular Islamic calendar used in Islamic Date Calculator is an arithmetic approximation of the observational Islamic calendar. It can differ from the actual observed calendar by one to two days because the observational calendar depends on physical sighting of the crescent moon, which varies by geographic location and weather conditions. For historical dates and general planning, the tabular calendar is sufficiently accurate and widely used in astronomy and computing. Saudi Arabia and many Muslim-majority countries use the Umm al-Qura calendar for civil purposes, which may differ slightly from the tabular system used here.
What are the twelve months of the Islamic calendar?
The twelve months of the Islamic calendar are Muharram, Safar, Rabi al-Awwal, Rabi al-Thani, Jumada al-Ula, Jumada al-Thani, Rajab, Shaban, Ramadan, Shawwal, Dhul Qadah, and Dhul Hijjah. Four of these are considered sacred months (Muharram, Rajab, Dhul Qadah, and Dhul Hijjah) during which warfare was traditionally prohibited. Ramadan is the month of fasting. Shawwal begins with Eid al-Fitr. Dhul Hijjah contains the Hajj pilgrimage period and concludes with Eid al-Adha. Each month begins with the sighting of the new crescent moon, though the tabular calendar uses mathematical rules for calculation.
Does a leap year affect date difference calculations?
Yes, leap years add February 29, extending the year to 366 days and affecting any date range that spans that date. A period from January 1 to December 31 covers 365 days in a regular year but 366 in a leap year. Similarly, 'one year from February 28' in a non-leap year is February 28, but in a leap year the next day (February 29) also exists, so applications must define whether 'one year later' maps to February 28 or February 29. Financial instruments like bonds and loans use specific day-count conventions (Actual/360, Actual/365, Actual/Actual) to handle these edge cases consistently.
Does Islamic Date Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.
What inputs do I need to use Islamic Date Calculator accurately?
Each field is labelled with the required unit (metric or imperial). Gather your source values before starting — for example, a weight measurement in kilograms, a distance in metres, or a dollar amount — and enter them exactly as measured. The formula section on this page lists every variable and explains what each represents.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy