Frequently asked questions
The questions people ask most about calendars — how they work, why they disagree, and how to read them here. General interest, not a claim of authority; where a date is an estimate, the site says so.
The basics
What calendar does the world use today?
Most of the world runs its civil life on the Gregorian calendar, the solar calendar introduced in 1582 and now the international standard for business and government. It is far from the only one in use, though: the Hebrew, Islamic, Persian, Chinese, Hindu and Ethiopian calendars, among many others, remain in daily religious and cultural use — often written alongside the Gregorian date.
How many calendars are there?
There is no fixed number. Dozens are in active civil or religious use around the world, and many more are historical, regional, or reconstructed. All Calendars Now tracks 61 at once, grouped into ten families — from the living calendars of faith and state to the reckonings of invented and fictional worlds.
Why do calendars show different years for the same day?
Because each counts from a different epoch — its own year zero. The Gregorian year counts from the traditional birth of Christ, the Islamic year from the Hijra in 622 CE, the Hebrew year from a calculated date of Creation, and Unix time from the first of January 1970. The day is the same; only the starting line differs.
Why do calendars matter?
Because a calendar is an agreement about when, and almost nothing people do together works without one. Farming needs it first — Egypt keyed its year to the rising of Sirius because that predicted the Nile flood — and religion needs it absolutely, since a fast or a feast means nothing unless everyone keeps it on the same day. Then the state: taxes fall due, terms of office begin and end, debts mature and courts sit, all on dates somebody had to fix. That is why setting the calendar has always been a form of power. Rome’s priests controlled when an extra month was inserted and used it to stretch the terms of magistrates they favored, until Caesar ended the practice by fixing the length of the year. Gregory’s reform of 1582 was adopted across Catholic Europe within a year and refused elsewhere for centuries — Britain held out until 1752, Russia until 1918, Greece until 1923 — because accepting a pope’s arithmetic was never only arithmetic.
Can a calendar be changed by decree?
The arithmetic can. The habit is harder. Revolutionary France replaced the week with ten-day décades in 1793, which gave working people one rest day in ten instead of one in seven, and the republic abandoned it after twelve years. The Soviet Union tried the opposite in 1929, staggering a continuous week so that factories never stopped, and gave that up too — it had quietly abolished the shared day off, and with it the idea of everyone being free at the same time. Both reforms were arithmetically sound and both failed for the same reason: a calendar is only worth keeping to the extent that other people are keeping it.
What is today’s date?
That depends entirely on which calendar you ask — which is the whole point of this site. Right now it is one date on the Gregorian calendar, another on the Hebrew, another again on the Chinese, and All Calendars Now shows them all at once for the very same instant.
How calendars work
What is the difference between a lunar, solar, and lunisolar calendar?
A solar calendar follows the sun and the seasons, like the Gregorian. A lunar calendar follows the phases of the moon and makes no correction to the solar year, so it drifts about eleven days earlier each year — the Islamic calendar is the classic case. A lunisolar calendar follows the moon but inserts a leap month every few years to keep its months in step with the seasons, as the Hebrew, Chinese and Hindu calendars do.
What is a leap year, and why is it needed?
The Earth takes about 365.2422 days to circle the sun — not a whole number. A calendar of exactly 365 days would slip against the seasons by a day every four years, so most solar calendars practice intercalation: adding a leap day, or in lunisolar calendars a whole leap month, to catch up.
Why does the Islamic date fall earlier every year?
Because it is a pure lunar calendar of twelve lunar months with no leap month. Twelve synodic months run about eleven days short of a solar year, so each Islamic year begins roughly eleven days earlier against the Gregorian calendar, cycling through all the seasons over about 33 years.
What is an epoch?
An epoch is the zero point a calendar counts from — the moment it calls the beginning. The Hijra is the epoch of the Islamic calendar, the first of January 1970 is the epoch of Unix time, and the Battle of Yavin is the epoch of the Star Wars calendar.
Reading this site
How does this site work out all these dates?
Everything is computed live in your browser, in your own time zone, for a single shared instant. The living calendars come straight from your browser’s own internationalization engine, so they match your operating system exactly; the historical, indigenous, invented and fictional ones are computed from published formulas.
Are these dates accurate?
The real calendars are exact. A few are approximate by nature and say so on their own page — the French Republican and Baháʼí years turn on the astronomical equinox, and the Maya and Aztec counts rest on the standard correlations. The fictional calendars are anchored to where their own canon places year zero, and those anchors are labeled estimates.
Why do the fictional calendars drift instead of matching our years?
Because a second is a second everywhere. This site never speeds up a clock to force another world’s day to end at our midnight, so a world whose day or year is a different length simply drifts against ours. That is the honest result — and, we think, the more interesting one.
Can I see the date somewhere other than where I am?
Yes. The line under the date names where the page is being read from — tap it and pick any of 163 cities. Everything follows: the clock, all 61 calendars, sunrise and sunset, and the daylight left, all as that city sees them. A city rather than a time zone, because half of what changes with location is astronomical and needs a point on the globe, not just an offset. Choose one far enough east or west and the date itself changes, since only two or three dates exist on Earth at any moment. The list says which ones those are: a +1 or −1 beside a city’s clock means it has already turned to tomorrow’s date, or is still on yesterday’s, measured from wherever you are reading. Between the far ends — Kiritimati and Pago Pago — the gap reaches a full two days.
Why did the Hebrew date change in the evening?
Because that is when the Hebrew day begins. Hebrew, Islamic and Baháʼí days run sunset to sunset rather than midnight to midnight, so between sunset and midnight their date is already the one the rest of us will call tomorrow’s. This site applies that boundary whenever it can work out where you are, and each of those three cards says which side of sunset it is on. Best case that is a city you picked or a fix from your device. Failing both, it uses a city standing in for your time zone and names it on the card — sunset there runs a median of seven minutes from your own, against the four and three quarter hours that a midnight boundary was out by. Only where no city in the list shares your zone does it fall back to midnight, and the card says so rather than quietly showing the wrong day.
Can I convert a specific date instead of seeing right now?
Yes. The date converter lets you pick any day on the Gregorian calendar and read it at once in every system here — real, invented and fictional.
Particular calendars
Why are there two Buddhist calendars here?
Because Thailand keeps two, and they answer different questions. The Thai Buddhist calendar is the civil one — the date on official documents, Gregorian in structure with the year counted from the Buddha’s passing, so 2026 CE is 2569 BE. The Thai Lunar calendar is the liturgical one, twelve numbered lunar months that fix Vesak, Asalha Bucha, Magha Bucha and the uposatha days. They share an era and nothing else: one moves with the sun, the other with the moon.
When is Vesak, and why does the date move so much?
Vesak — Visakha Bucha in Thailand — falls on a full moon in May or early June, because it is set by a lunar calendar rather than a solar one. The lunar year runs about eleven days short, so the date walks backward through the season until an intercalary month is added to pull it forward again. In one of those leap-month years the observances shift month as well: Visakha Bucha moves to month 7 and Asalha Bucha to a second month 8, which is why two consecutive years can look inconsistent and both be right.
What is today’s date in the Mayan calendar?
The Maya kept time in several interlocking counts: the Long Count of days since a mythic creation in 3114 BCE, the 260-day sacred Tzolkʼin, and the 365-day Haabʼ. All Calendars Now shows all of them for the current instant, using the standard GMT correlation.
What year is it in the Hebrew, Chinese, or Islamic calendar?
Each counts from its own epoch, so the numbers differ widely. The Hebrew year sits in the 5780s, counted from Creation; the Islamic year in the 1440s, counted from the Hijra; and the Chinese year is named by the sexagenary cycle of stems and branches rather than simply numbered. All Calendars Now shows each one live.
What is the Star Wars or Discworld date?
The site places the current instant on their in-universe calendars. The Star Wars date is counted in years before or after the Battle of Yavin on the Galactic Standard Calendar; the Discworld date runs on its own eight-day week, with the octeday. Both anchors are, of course, affectionate estimates.
What was the French Republican calendar?
A radical reform of the 1790s that broke the year into twelve thirty-day months of three ten-day décades, opened at the autumn equinox, with five or six festival days at the end. It lasted about twelve years before Napoleon abolished it. All Calendars Now still keeps its time.
The sun and moon
Why is it still light after the sun has set?
Because the upper air goes on scattering sunlight after the sun itself has dropped below the horizon. That lingering light is twilight, and it is graded by how far the sun has fallen: civil twilight to 6° below, nautical twilight to 12°, astronomical twilight to 18°, after which it is properly night.
How long does twilight last?
It depends on where you are and the time of year. Near the equator the sun drops almost straight down and civil twilight can be over in twenty minutes; the further from the equator, the more slanting the descent and the longer the light lingers. Far enough north in summer the sun never falls 6° below the horizon at all, and the twilight simply runs until dawn.
What does this site mean by daylight left?
Two figures, and it shows both. The first counts to sunset — the sun genuinely above the horizon. The second carries on to the end of civil twilight, which is the light you can actually see by. The gap between them is small in the tropics and large toward the poles: about twenty minutes in Singapore, over an hour in Helsinki. Once the sun has set only the twilight figure remains, and overnight it becomes the wait for sunrise.
Why does it say midnight sun, or polar night?
Because at that place, on that date, the sun does not set or does not rise. Above the Arctic and Antarctic circles this lasts weeks or months rather than a day: Longyearbyen has no sunset from April to late August, and no sunrise from late October to mid-February. There is a third state between them — the sun sets, but civil twilight never ends, so it stays light all night. That one holds around 61 to 65 degrees at midsummer, which is why Tromsø reads it in late July with the sun setting at 23:58.
Why does this site ask for my location?
Only to work out sunrise and sunset where you are, which a time zone alone cannot tell it. Your coordinates are used on your device and never sent anywhere, and everything else on the site — every calendar, every clock — works perfectly well without them. You do not have to say yes: without permission the sun row falls back to a city standing in for your time zone and says which one, which is close enough to be useful and honest about not being exact. Note that this is asked of your browser, not worked out from your IP address, so a VPN does not change the answer — and equally, a VPN cannot supply one.
What is the moon phase shown at the top?
The moon’s current place in its cycle from one new moon to the next, with the percentage of its face lit. That cycle — the synodic month of about 29.5 days — is what every lunar and lunisolar calendar on this site is built on.
What does “waxing gibbous” mean?
Waxing means the lit part is still growing, on its way from new moon to full. Gibbous means more than half the face is lit but it is not yet full — from the Latin for a hump, which is what that lopsided shape looks like. So a waxing gibbous moon is the fat, not-quite-full moon of the few nights before the full one.
Why is a quarter moon half lit?
Because the quarter counts how far the moon has traveled through its cycle, not how much of it you can see. At first quarter it is a quarter of the way from one new moon to the next — and from Earth that works out as exactly half the face lit.
Why do so many calendars start their months at the new moon?
Because a month was originally a moon. The cycle from one new moon to the next is about 29.5 days, plain to see and the same for everybody, which made it the obvious unit long before anyone could measure a year accurately. The Hebrew, Islamic, Chinese and Hindu calendars all still open their months on it — the Islamic traditionally on the first thin crescent sighted after it.
Beyond Earth
What time is it on Mars right now?
This site shows it. Coordinated Mars Time is mean solar time on the Mars prime meridian — the Martian answer to UTC — and beside it runs the Mars Sol Date, a continuous count of sols from 29 December 1873 that works the way our Julian Day does. The algorithm is Allison and McEwen’s, the one behind NASA’s own Mars24 clock.
Why does Mars time drift against ours?
A Martian day — a sol — is 24 hours 39 minutes 35 seconds, about forty minutes longer than ours. Nothing here is rescaled to hide that: a second is a second everywhere, so a clock kept on sols simply gains forty minutes a day. Mission teams who live on Mars time start work later each day until they have gone right round and come back into step with Earth, which takes about five weeks.
Is there a calendar for Mars?
Several have been proposed; the most worked-out is the Darian calendar, drawn up by Thomas Gangale in 1985. Twenty-four months of 28 sols with every sixth docked to 27, giving 668 sols against the 668.59 Mars actually takes, and a seven-sol week named for the same planets ours is. Its months alternate Latin and Sanskrit names for the zodiac. Nobody keeps it, because nobody lives there, so this site files it with the invented reckonings rather than the calendars in use.
What is the date on the Moon?
The Moon keeps no clock, and that is the interesting part — a lunar solar day is about 29.5 Earth days, so twenty-four lunar hours would each run nearly thirty of ours, and nobody reckons that way. What lunar observers use instead is a count and an angle: the Brown Lunation Number, running continuously from the first new moon of 1923, and selenographic colongitude, the longitude of the sunrise line creeping across the surface at 12.2 degrees a day. That angle is the Moon’s own time of day, and it tells an observer which craters are catching low light tonight.
Why does Jupiter have three clocks?
Because it is not solid and does not turn as one body. System I covers the fast equatorial belt at 9h 50m 30s, System II the temperate latitudes on either side at 9h 55m 41s, and System III the magnetic field at 9h 55m 30s — the one the IAU treats as the planet’s true rotation, since the field is anchored to the interior rather than the visible cloud. Each reports the longitude facing Earth this moment, which is how an observer knows when the Great Red Spot comes round.
Clocks and standards
What is the difference between GMT, UTC, and atomic time?
GMT is the old mean solar time at Greenwich. UTC is the modern civil standard, kept in step with the turning Earth by the occasional leap second. Atomic time (TAI) is the raw count of the world’s atomic clocks, which takes no leap seconds at all — and so now runs 37 seconds ahead of UTC.
Why are some time zones offset by half or three quarters of an hour?
Because a time zone is a political choice, not an astronomical one, and some places set theirs to fit their own noon rather than a round hour from Greenwich. Thirteen offsets in use are not whole hours, among them India and Sri Lanka at +05:30, Nepal at +05:45, Iran +03:30, Afghanistan +04:30, Myanmar +06:30, central Australia +09:30, Newfoundland −03:30, the Marquesas −09:30. The rarest is Eucla in Western Australia at +08:45, and the Chatham Islands keep +12:45. Lord Howe Island goes further still and shifts its clock by half an hour for daylight saving rather than a whole one.
Why is standard time barely a century old?
Because until trains, nobody needed it. Every town kept its own noon, set by the sun directly overhead, so Bristol ran about ten minutes behind London and neither had reason to care. Railways made that intolerable and then dangerous, since a timetable is useless if each station reads a different clock — so British railways simply adopted London time from the 1840s, decades before Parliament made it law in 1880. American railroads did the same on their own authority in November 1883, resetting the country to four zones on a day the newspapers called the day of two noons; Congress only caught up in 1918. The international scheme of 24 zones measured from Greenwich was agreed in 1884. So the world’s time zones are younger than the telephone, and they exist for engineering reasons rather than astronomical ones — which is still true of what succeeded them, since shipping, markets and networks now run on atomic time.
What is a Julian Day Number?
A continuous count of days used by astronomers since the first of January 4713 BCE. It sidesteps every calendar’s months and leap rules, which makes measuring the interval between two dates trivial. Despite the shared name it has nothing to do with the Julian calendar.
What is Unix time?
The number of seconds elapsed since midnight UTC on the first of January 1970 — the way most computers store a moment internally. All Calendars Now shows it ticking up second by second.
What is Swatch Internet Time?
A 1998 scheme that split the day into 1000 “.beats” on a single worldwide clock with no time zones, so that @500 is the same instant everywhere. A curiosity now, but All Calendars Now keeps it running.
Want the vocabulary behind the answers? Browse the glossary of timekeeping, or see this moment in every calendar at once.