Why does the Islamic calendar move each year?

Because it is purely lunar, with twelve lunar months and no leap month. A Hijri year of about 354 days is roughly eleven days shorter than a Gregorian one, so every Hijri date shifts earlier each year and works through all four seasons in about thirty-three years.

Why is Chinese New Year on a different date each year?

Because the Chinese calendar is lunisolar and its months begin at the new moon, so the year starts on a date that shifts against the Gregorian calendar. It always falls between 21 January and 20 February, with the exact day set by the lunar cycle.

When did the world adopt the Gregorian calendar?

Gradually, over more than three centuries. Catholic Europe changed in 1582, Britain and its colonies in 1752, Japan in 1873, China in 1912, Russia in 1918 and Greece in 1923. International trade, not religious agreement, is what eventually made it universal.

Calendar Systems Compared

Q: What is a lunisolar calendar?

One whose months follow the moon while its years stay aligned with the sun. Since twelve lunar months fall about eleven days short of a solar year, an extra month is added seven times in every nineteen years. The Hebrew and Chinese calendars both work this way.

Today has more than one date. The Gregorian calendar supplies the one on your screen, but the same twenty-four hours also carry a Hijri date, a Hebrew date and a Chinese date, and none of those will fall on the same numbered day next year that it does this year. The world's calendar systems are built on different astronomical foundations, count from different starting points, and drift against one another in ways that are entirely predictable and thoroughly confusing.

This article explains how the Gregorian calendar came to dominate civil dating, how the main lunar and lunisolar calendar systems differ from it, and why their dates cannot be made to line up. The Gregorian value for today, with its weekday and ordinal position, is on the today's date page.

How Did The Gregorian Calendar System Become Dominant?

Through a mixture of astronomy, empire and commerce. It began as a correction to the Julian calendar system, issued in 1582 by Pope Gregory XIII, and spread outward from Catholic Europe over the following three and a half centuries until it became the common language of international business.

Adoption was slow and often reluctant. Protestant and Orthodox states resisted a papal reform for generations, so Britain and its colonies did not change until 1752, Japan adopted the Gregorian calendar in 1873, China in 1912, Russia in 1918 and Greece in 1923. What eventually settled the matter was not theology but trade: shipping, telegraphy and international finance all needed one agreed calendar system, and the Gregorian was already the most widely used. Several Orthodox churches still keep the older Julian calendar system for liturgical purposes, which is why some religious feasts fall thirteen days later than the civil date suggests.

What Is A Lunisolar Calendar?

A lunisolar calendar system keeps its months tied to the phases of the moon while keeping its years tied to the sun. Because twelve lunar months run about eleven days short of a solar year, such calendar systems insert a whole extra month every few years to stop the seasons sliding.

The astronomy behind this is fixed and unforgiving. A lunar month averages roughly 29.53 days, so twelve of them come to about 354 days against the solar year's 365. Adding a thirteenth month seven times in every nineteen years brings the two back into step almost exactly, an equivalence known since antiquity as the Metonic cycle. Calendar systems that make this correction, including the Hebrew and Chinese ones, keep their festivals in the right season. A calendar system that declines to make it, as the Hijri one does, lets its months travel through the seasons instead.

The Islamic Or Hijri Calendar

The Hijri calendar system is purely lunar. Twelve lunar months make a year of 354 or 355 days, with no intercalary month, so every Hijri date falls about eleven days earlier in the Gregorian year than it did the year before and completes a full circuit of the seasons in roughly thirty-three years.

Its epoch is the Hijra, the migration of the Prophet Muhammad from Mecca to Medina in 622 CE, and years are counted from there as AH. Months traditionally begin with the first sighting of the new crescent moon, which means a month's start can differ by a day between countries depending on observation and on whether calculated tables are used instead. This is why Ramadan moves steadily through the Gregorian year, appearing in winter for some years and in high summer for others, and why announced start dates are not always identical everywhere. Saudi Arabia has long used Hijri dates in official documents, frequently printed alongside their Gregorian equivalents.

The Hebrew Calendar

The Hebrew calendar system is lunisolar, with months beginning at the new moon and a leap month, Adar I, inserted seven times in each nineteen-year cycle. An ordinary year runs 353 to 355 days and a leap year 383 to 385, the variation coming from rules that prevent certain festivals from falling on inconvenient weekdays.

Its epoch is anno mundi, the traditional date of creation, corresponding to 3761 BCE, so the Hebrew year number runs some 3,760 ahead of the Gregorian one. The calendar has been governed by calculation rather than observation for well over a thousand years, which makes its dates predictable far into the future. Its week is the source of a convention still visible in Western calendars: the Sabbath falls on the seventh day, Saturday, making Sunday the first day of the week, a legacy examined in why weeks start on Monday or Sunday.

The Chinese Calendar

The Chinese calendar system is also lunisolar. Months begin at the new moon as observed from a defined meridian, and a leap month is added roughly every three years, giving years of about 354 days or, when a thirteenth month is inserted, about 384.

Chinese New Year therefore has no fixed Gregorian date, falling somewhere between 21 January and 20 February. Alongside the lunisolar year runs the sexagenary cycle, a sixty-year sequence formed by pairing ten heavenly stems with twelve earthly branches; the twelve branches give the familiar zodiac animals. China has used the Gregorian calendar for civil purposes since 1912, but the traditional calendar still fixes festivals and remains in daily use for that purpose, a pattern repeated across East Asia. Japan adds a further layer, numbering years by imperial era alongside the Gregorian year, so official documents may carry both.

Why Calendar Systems Never Line Up

Because these calendar systems are measuring different things from different starting points. A solar calendar tracks the earth's orbit, a lunar one tracks the moon's phases, and a lunisolar one compromises between the two, so no fixed offset can convert between them.

Four differences do most of the damage:

Different year lengths: 365 or 366 Gregorian days, about 354 Hijri days, and 353 to 385 in the lunisolar systems.
Different epochs: each calendar counts from its own starting event, so the year numbers bear no simple relation to one another.
Different day boundaries: the Hebrew and Islamic days begin at sunset, not midnight, so a date can change while the Gregorian one has not.
Different rules for insertion: the Gregorian leap year rule adds a day, the lunisolar systems add a whole month, and the Hijri calendar adds nothing at all.

For this reason astronomers avoid converting between calendar systems altogether, working instead from the continuous count described in day of year and Julian day, which is indifferent to the calendar system a date was originally written in. The related question of how the same Gregorian date is written differently around the world is covered in date format by country.

Conclusion

The Gregorian calendar won the civil world by being useful for trade rather than by being astronomically superior, and the older calendar systems continue alongside it. The Hijri year is purely lunar and drifts eleven days a year against it; the Hebrew and Chinese calendars are lunisolar and correct themselves with a thirteenth month seven times in nineteen years. Different year lengths, different epochs and different day boundaries mean no two of them can be aligned by a fixed rule. See today's Gregorian date, weekday and ordinal position on the today's date page, work out spans with the date calculator, or browse everything at datetoday.now.

Calendar Systems: Dates In Hijri, Hebrew And Chinese Years