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Calendar Converter | Gregorian, Julian, Islamic, Hebrew & Unix Time

An astronomical, historical and mathematical guide for converting dates between Gregorian, Julian Day (JD), Islamic, Hebrew, Persian Jalali, Mayan, ISO-8601 and Unix systems.

Çoklu Takvim ve Tarih Dönüştürücü

Jülyen Günü (JD) köprüsü üzerinden dünya takvimleri, antik sistemler ve zaman damgaları arasında çift yönlü hassas dönüşüm.

Miladi1 Ocak 2026
Hicri12 Recep 1447
Hafta GünüPerşembe
Jülyen Günü (JD)2461041.50
Miladi Takvim (Gregorian)
Normal YılPerşembe
Uluslararası standart güneş takvimi.
:
2026-01-01 00:00:00
Hicri Takvim (Islamic / Kameri)
Artık Yıl (355g)al-khamis
1 Muharrem 1 H (16 Temmuz 622) başlangıçlı saf ay takvimi.
12 Recep 1447 H
Jülyen Takvimi (Eski Usul / Rumi Kök)
Normal Yıl
M.Ö. 46 Julius Caesar reformu (her 4 yılda bir artık yıl).
19 Aralık 2025 (Jülyen)
Celali / İran Takvimi (Şemsi Hicri)
Artık YılPanjshanbeh
Ömer Hayyam kökenli, 2820 yıllık döngüye sahip hassas güneş takvimi.
12 Dey 1404 (Şemsi)
İbrani Takvimi (Hebrew / Yahudi)
12 Aylık Normal Yıl354 Gün
M.Ö. 3761 başlangıçlı, 19 yıllık Meton döngüsüne sahip luni-solar takvim.
12 Teveth 5786

Guide & Content



Comprehensive Calendar Converter Guide: Civil, Astronomical & Computer Systems

Throughout human history, civilizations, religions, and sciences have devised ingenious calendar systems based on the solar year, lunar cycles, or a combination of both. While the Gregorian calendar is the undisputed universal standard for international civil life and commerce today, historical records, religious observances, astronomy, and computer architectures rely on an intricate variety of date reckoning systems.
Our Online Calendar Converter provides precise, bidirectional conversions across the world's major historical, civil, astronomical, and computer calendars.


1. Gregorian Calendar

Proclaimed by Pope Gregory XIII in 1582, the Gregorian calendar corrected the accumulating drift between the ancient Julian calendar and the solar equinox by dropping 10 days (Thursday, October 4, 1582 was immediately followed by Friday, October 15, 1582).
  • Global Adoption: Catholic nations adopted it immediately, but Great Britain and its American colonies did not switch until September 1752 (where Sept 2 was followed by Sept 14), and other countries adopted it as late as the early 20th century.
  • The Leap Year Rule: Every year divisible by 4 is a leap year, except end-of-century years divisible by 100, which are common years unless also evenly divisible by 400 (e.g., 1600 and 2000 were leap years; 1700, 1800, and 1900 were not).
  • Accuracy: The mean Gregorian year is
    365.2425365.2425
    
    days, accumulating only 1 day of discrepancy against the solar tropical year (
    365.24219878365.24219878
    
    days) every ~3,300 years.


2. Astronomical Julian Day (JD) and Modified Julian Day (MJD)

Unlike historians who deal with named months and leap years, astronomers and astrophysicists require simple arithmetic with continuous time intervals.
  • Epoch: Julian Day numbers simply count the continuous days and fractions elapsed since noon on Monday, January 1, 4713 B.C.E. (Julian proleptic calendar).
  • Why Noon? Because astronomers traditionally work through the night; setting the date change at noon prevents calendar dates from rolling over in the middle of an observational night.
  • Modified Julian Day (MJD): Defined as
    MJD=JD−2,400,000.5MJD = JD - 2,400,000.5
    
    , counting days elapsed since midnight UTC on November 17, 1858. MJD is widely used in tracking artificial Earth satellite orbital elements.


3. Julian Calendar

Introduced by Julius Caesar in 46 B.C., the Julian calendar assumes a flat year length of
365.25365.25

days with a leap year every four years without century exceptions. As a result, it gains roughly 1 day of error against the solar seasons every 128 years. It serves as the baseline for all European historical dates prior to October 1582.


4. Islamic (Hijri) Calendar

The Islamic calendar is strictly lunar, consisting of 12 alternating months of 30 and 29 days (
354354

days per common year).
  • The Epoch (Hijrah): Reckoned from the migration of Prophet Muhammad (PBUH) from Mecca to Medina on July 16, 622 C.E. (Julian), designated 1 Muharram 1 A.H.
  • 30-Year Intercalary Cycle: To maintain lunar synchronization, 11 leap years of 355 days occur within every 30-year cycle (intercalary day added to Dhu al-Hijjah).
  • Seasonal Drift: Because an Islamic year is approximately 11 days shorter than a solar year, Islamic holy months (such as Ramadan) cycle backward through all four solar seasons every 33 years.


5. Persian (Jalali / Solar Hijri) Calendar

Adopted in modern form in 1925, the Persian calendar is the most accurate solar calendar in active use today. Its astronomical roots trace back to the 11th-century Sultanic calendar calculated by Omar Khayyam.
  • The year begins precisely at the vernal equinox (Nowruz). The first 6 months have 31 days, the next 5 have 30 days, and the final month has 29 (30 in leap years).
  • Its sophisticated 2820-year leap cycle achieves such phenomenal accuracy that it accumulates an error of one day only once every 3.8 million years.


6. Hebrew (Jewish) Calendar

The Hebrew calendar is luni-solar: months are aligned with the lunar phases, while years are synchronized with the solar agricultural cycle. To bridge the 11-day gap between 12 lunar months and the solar year, a 19-year Metonic cycle is employed, adding a 29-day leap month (Adar II) in years 3, 6, 8, 11, 14, 17, and 19. The epoch starts at sunset on October 7, 3761 B.C.E.


7. Mayan Calendars

The ancient Maya civilization utilized three parallel cyclic systems:
  1. Long Count: A linear tally of days elapsed since August 11, 3114 B.C.E., structured hierarchically like an IP address: baktun.katun.tun.uinal.kin (
    1 kin=1 day,1 uinal=20 days,1 tun=360 days,1 katun=7,200 days,1 baktun=144,000 days1\text{ kin}=1\text{ day}, 1\text{ uinal}=20\text{ days}, 1\text{ tun}=360\text{ days}, 1\text{ katun}=7,200\text{ days}, 1\text{ baktun}=144,000\text{ days}
    
    ).
  1. Haab: Civil 365-day solar cycle consisting of 18 named periods of 20 days plus 5 uncounted days (Uayeb).
  1. Tzolkin: Sacred 260-day ritual cycle generated by pairing 20 day names with 13 numbers.


8. Computer & Operating System Time Formats

Unix Time (Epoch 1970):

Universal computer standard measuring time as the signed number of seconds elapsed since 00:00:00 UTC on January 1, 1970.
  • Year 2038 Problem: Legacy 32-bit signed integers will overflow on January 19, 2038 at 03:14:07 UTC. Modern operating systems mitigate this by transitioning to 64-bit timestamps.

Excel Serial Date Numbers & The 1900 Leap Year Bug:

  • Windows PC Excel: Treats January 1, 1900 as day 1.0. To maintain compatibility with Lotus 1-2-3, Microsoft intentionally copied a deliberate bug that treats 1900 as a leap year (February 29, 1900), which it was not. Consequently, all Excel serial dates after February 28, 1900 are offset by +1 day.
  • Macintosh Excel (1904 System): Avoids this bug by starting its epoch at January 1, 1904 as day 0.0.


9. ISO-8601 Calendar Week and Day

The International Standard ISO 8601 defines date formats (YYYY-MM-DD) and the commercial "Week Date" system (YYYY-Www-D). Week 01 of any year is defined as the week containing the first Thursday of that year (or containing January 4). Weeks begin on Monday.

📚 References & Academic Bibliography
  1. Meeus, Jean: Astronomical Algorithms. Richmond: Willmann-Bell, 1991. ISBN: 0-943396-35-2 — The authoritative international guide for computational positional astronomy.
  1. P. Kenneth Seidelmann (ed.): Explanatory Supplement to the Astronomical Almanac. University Science Books, 1992. ISBN: 0-935702-68-7 — Foundational text on computational geodesy, ephemerides, and calendar transformations.
  1. IMCCE (Institut de mécanique céleste et de calcul des éphémérides): Paris Observatory Celestial Mechanics and Calendar Portal.
  1. Fourmilab Switzerland: John Walker's Open-Source Calendar Converter Architecture.
  1. ISO (International Organization for Standardization): ISO 8601 Date and Time Format Standards.

İşte Takvim Dönüştürücü (Calendar Converter) aracı için hazırlanan, hem yasal hem de astronomik/dini hassasiyetleri kapsayan Türkçe ve İngilizce Sorumluluk Reddi (Disclaimer) metinleri:


⚠️ Legal Disclaimer & Terms of Use

General Informational Purpose: The calendar conversions provided on this page are computed using internationally recognized astronomical and mathematical algorithms (including Jean Meeus's Astronomical Algorithms, P. K. Seidelmann's Explanatory Supplement to the Astronomical Almanac, and ISO 8601 standards). All calculations are provided strictly for general informational, educational, and historical research purposes.
📌 Key Considerations:
  • Historical Adoption Variance: The adoption of the Gregorian calendar occurred at different dates across nations and jurisdictions (e.g., October 1582 in Catholic Europe, September 1752 in Great Britain and the American colonies, and 1918–1926 in Eastern Europe and Turkey). Dates preceding October 1582 are extrapolated mathematically as proleptic dates.
  • Religious Calendars & Lunar Sightings: Islamic (Hijri) and Hebrew conversions utilize arithmetic and cyclical models. Actual religious observances (such as Ramadan, Eid, or Rosh Hashanah) may vary by ±1 day based on regional sightings of the lunar crescent by official religious authorities.
  • Limitation of Liability: This tool is provided "as is" without warranties of any kind. It should not be used as the sole reference for binding legal statutes of limitations, court filing deadlines, or official financial settlement dates where jurisdictional statutory codes prevail.

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