The Gregorian and Muslim (Hijri) calendars share more structural DNA than most people realize. Both divide the year into 12 months, both use a seven-day week, and both count years forward from a single defining historical event. The differences in how they track time are significant, but the underlying architecture is remarkably similar.
Both Use 12 Months and 7-Day Weeks
The most obvious similarity is that both calendars split the year into exactly 12 named months. The Gregorian calendar runs from January through December, while the Hijri calendar runs from Muharram through Dhul Hijjah. This 12-month structure isn’t a coincidence. Both systems descend from older Semitic and Mediterranean calendaring traditions that used the lunar cycle (roughly 29.5 days) as the basic unit of a “month,” and 12 of those cycles as a year.
Both calendars also operate on a seven-day week. The shared week structure traces back to ancient Near Eastern traditions that influenced Judaism, Christianity, and Islam alike. Friday holds special significance in Islam, Sunday in most Christian traditions, and Saturday (the Sabbath) in Judaism, but the seven-day cycle itself is identical across all three. Early Islamic tradition placed particular importance on Monday as well. The Prophet Muhammad is reported to have noted that Monday was the day of his birth and the day the Quran was first revealed.
Each Calendar Starts From a Defining Event
Both systems anchor Year 1 to a moment of deep religious or cultural significance. The Gregorian calendar counts forward from the estimated birth year of Jesus (though modern scholarship places his actual birth a few years earlier). The Hijri calendar begins with the migration of the Prophet Muhammad from Mecca to Medina in 622 CE, an event known as the Hijra. Day one of the Islamic calendar, 1 Muharram of Year 1, corresponds to July 16, 622 CE on the Julian calendar.
This approach to timekeeping, picking a pivotal event and counting forward, is called an epoch. It gives each calendar system a shared reference point that an entire civilization can organize around. In both cases, that reference point is religious in origin, reflecting how deeply intertwined faith and civic life were when these systems took shape.
Both Systems Use Leap Year Corrections
Neither calendar lands on a perfectly round number of days per year, so both need periodic corrections to stay accurate. The Gregorian calendar adds one day every four years (February 29), with additional rules skipping leap years in most century years. This corrects for the fact that a solar year is about 365.242 days long, not a clean 365.
The Hijri calendar faces the same problem in miniature. A lunar year is approximately 354.37 days, but individual months alternate between 29 and 30 days, producing a standard year of 354 days. To make up the gap, the tabular (arithmetic) version of the Islamic calendar inserts one extra day into the last month of the year during 11 out of every 30 years. Those leap years fall at positions 2, 5, 7, 10, 13, 15, 18, 21, 23, 26, and 29 within each 30-year cycle. The logic is the same as the Gregorian system: add small corrections at regular intervals so the calendar doesn’t slowly drift away from the astronomical cycle it’s tracking.
Where the Similarities End
Understanding the similarities is easier when you also see where the two systems diverge, because the differences highlight just how much the calendars share despite their distinct foundations.
The biggest difference is what each calendar tracks. The Gregorian calendar follows the sun. Its year of roughly 365.242 days matches the time Earth takes to orbit the sun, which is why seasons always fall in the same months. The Hijri calendar follows the moon. Its year of about 354.37 days is nearly 11 days shorter than a solar year. That gap means Islamic months rotate backward through the Gregorian calendar, cycling through all four seasons over a span of roughly 33 years. Ramadan, for example, can fall in summer one decade and winter the next.
This seasonal drift is not a flaw. It’s a deliberate feature. Pre-Islamic Arabs had used an intercalary system (adding an extra month periodically) to keep their lunar calendar aligned with the seasons, similar to what the Jewish calendar still does. Islam discontinued that practice, producing a purely lunar calendar that moves independently of the solar cycle.
Converting Between the Two Calendars
Because the two calendars run at different speeds, converting a date from one to the other requires a bit of math. A rough conversion from a Hijri year to a Gregorian year works like this: multiply the Hijri year by 0.97, then add 622. Going the other direction, subtract 622 from the Gregorian year and multiply by 1.03.
For example, the Islamic year 1446 converts approximately to: 1446 × 0.97 + 622 = 2024.6, placing it in 2024-2025 CE. These formulas give approximate results. Because the two calendars accumulate their difference day by day rather than year by year, pinpointing an exact date (rather than just a year) requires more detailed tables or software that accounts for the specific month and day within each system.
The 0.97 multiplier reflects the ratio between the two year lengths. Every 33 Gregorian years, the Hijri calendar gains roughly one full extra year. That’s why the current Hijri year (in the mid-1440s) is so much higher than you’d get by simply subtracting 622 from the Gregorian year. Over 14 centuries, those extra 11 days per year have added up to about 42 additional Hijri years beyond what a simple subtraction would predict.
Shared Roots in Earlier Traditions
Many of the structural parallels between the two calendars trace back to shared ancestry in ancient Near Eastern timekeeping. The 12-month year, the seven-day week, and the concept of sacred months all predate both Christianity and Islam. The Jewish calendar served as a particularly important bridge. Two of the sacred months in pre-Islamic Arabia, Muharram and Rajab, appear to correspond to Jewish pilgrimage months (Tishri and Nisan). The Islamic fast on the 10th of Muharram (Ashura) may have originally aligned with Yom Kippur on the Jewish calendar.
These connections don’t make the calendars interchangeable, but they explain why two systems developed centuries apart on different astronomical principles ended up with so many features in common. The scaffolding, 12 months, 7-day weeks, leap corrections, a Year 1 tied to sacred history, is shared. What fills that scaffolding, whether the calendar chases the sun or the moon, is where each tradition made its own choice.