All Calendars Now

A glossary of timekeeping

Every calendar here leans on the same handful of ideas — leap months and epagomenal days, the Metonic cycle and the sankranti, epochs and eras. Here they are in plain language.

How Calendars Work

Lunisolar calendar: A calendar that follows the moon’s months but adds a leap month every few years to keep them aligned with the solar year — as the Hebrew, Chinese and Hindu calendars do.
Lunar calendar: A calendar of pure lunar months that makes no adjustment to the solar year, so it drifts about eleven days earlier against the seasons each year. The Islamic calendar is the great example.
Solar calendar: A calendar governed only by the sun and the seasons, like the Gregorian or Persian. Its months are a convenience rather than true lunar months.
Intercalation: The insertion of extra days, or a whole extra month, to keep a calendar in step with the sun or moon. A leap day is intercalation; so is the Hebrew leap month.
Leap month: A thirteenth month added to a lunisolar year — seven times every nineteen years in the Hebrew and Chinese calendars — to stop the lunar months sliding through the seasons. Also called an embolismic month.
Epagomenal days: A handful of days, usually five or six, added outside the regular months to complete the year — as in the Coptic, Ethiopian and ancient Egyptian calendars. The Aztec nemontemi and the Baháʼí Ayyám-i-Há are the same idea.
Décade: The ten-day week of the French Republican calendar, three to a month — a deliberate break with the seven-day week.
Proleptic calendar: A calendar extended backward to dates before it actually existed — as when a historian places an event in the “proleptic Gregorian” calendar, centuries before the 1582 reform.
Perpetual calendar: A reform in which every date falls on the same weekday every year, achieved by setting a day or two outside the week. The International Fixed and World calendars both do this.
Daylight saving time: Shifting the clock forward in summer so the light falls later in the day. Most of the world does not do it, and those that do disagree on the dates; Lord Howe Island is alone in shifting by half an hour rather than a whole one.

Sky & Cycles

Synodic month: The 29.53 days from one new moon to the next — the true lunar month that lunar and lunisolar calendars track.
Tithi: A lunar day: one thirtieth of a synodic month, the time the moon takes to gain 12° on the sun. Because that gain is uneven, a tithi runs anywhere from about 22 to 26 hours and does not line up with a solar day — which is why calendars built on it, across India and Southeast Asia, need rules for which civil day a tithi belongs to.
Metonic cycle: The near-coincidence, noted by Meton of Athens, that 19 solar years almost exactly equal 235 lunar months. It underlies the leap-month pattern of the Hebrew calendar and the reckoning of Easter.
Solar term: One of 24 points dividing the solar year in the Chinese calendar, marking the sun’s progress and fixing when leap months fall.
Sidereal day: A day measured against the distant stars rather than the sun — 23h 56m 04s, about four minutes short of a solar day, because Earth also moves along its orbit.
Precession: The slow ~26,000-year wobble of Earth’s axis, which shifts the equinox against the stars — the reason the sidereal zodiac has fallen roughly 24° behind the tropical one.
Equinox: The moment the sun crosses the celestial equator and day and night stand equal. Several calendars — Persian, Baháʼí, French Republican — open their year at one.
Solstice: The two moments each year when the sun reaches its furthest from the equator, giving the longest and shortest days. Several calendars open their year at one — the Wheel of Time’s Farede Calendar at midwinter, which also keeps a day outside the months at midsummer.
Lunar phase: How much of the moon’s lit face we can see, which changes as it rounds the Earth. The cycle is conventionally named in eight steps: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent — then new again, 29.5 days later.
Waxing and waning: Waxing is the half of the cycle when the lit part is growing, from new toward full; waning is when it is shrinking, from full back to new. In the northern hemisphere a waxing moon is lit on the right, a waning one on the left — and the other way about south of the equator.
Crescent: Less than half the face lit — the thin shape just after the new moon or just before the next. The first thin crescent after a new moon, the hilāl, is what traditionally begins a month in the Islamic calendar.
Gibbous: More than half lit but not yet full, from the Latin gibbus, a hump — the lopsided moon of the nights either side of full. Waxing gibbous comes before the full moon, waning gibbous after it.
First and last quarter: The two moments when exactly half the face is lit. The name confuses people reasonably often: the quarter counts the way through the cycle, not the amount lit — a quarter moon looks like a half.
New and full moon: At new moon the lit face is turned away and the moon is lost in the sun’s glare; at full it is opposite the sun and lit right across. Most lunar and lunisolar calendars open their months at one or the other, which is why the moon governs so much of this site.
Twilight: The light before sunrise and after sunset, when the sun is below the horizon but the upper air still catches and scatters its light. It is graded into three bands — civil, nautical and astronomical — by how far the sun has fallen.
Civil twilight: The first band, with the sun between the horizon and 6° below it. There is still enough light to read or work outdoors and only the brightest stars show — the span most people mean by “still light out.”
Nautical twilight: The sun between 6° and 12° below the horizon. Named for the sailor’s art: the horizon stays faintly visible while the stars are out, the one window in which a navigator can measure a star’s height against it.
Astronomical twilight: The sun between 12° and 18° below the horizon. The sky looks dark, but the last scattered sunlight still veils the faintest objects; below 18° is true night. At high latitudes in summer it never arrives at all.
Dawn and dusk: Dawn is the morning twilight and dusk the evening one — not the moments of sunrise and sunset themselves, though they are often used that way. Each takes the name of its band: civil dawn, nautical dusk, and so on.
Sankranti: The moment the sun enters a new sidereal zodiac sign, which begins each month of the Hindu solar and Vikram Samvat calendars — so those months run from 29 to 32 days.
Midnight sun: Above the Arctic or Antarctic circle in that hemisphere’s summer, the sun never sets at all — it circles the sky and dips toward the horizon without reaching it. Longyearbyen has no sunset from 19 April to 25 August.
Polar night: The winter counterpart: the sun never rises. It is rarely pitch dark, since twilight still washes the sky around noon for much of it, but the disc itself stays below the horizon — nearly four months of it at 78° north.
Central meridian longitude: The longitude of a rotating planet facing Earth at a given moment, written CML. On Jupiter it is quoted in three systems at once, because the equator, the temperate belts and the magnetic field all turn at different rates.
Sol: A Martian solar day — 24 hours 39 minutes 35 seconds, about forty minutes longer than ours. A clock kept on sols gains that much on Earth every day.
Lunation: One full cycle of the Moon from new to new, about 29.53 days. The Brown Lunation Number counts them continuously from the first new moon of 1923.
Terminator: The line dividing the lit half of a world from the dark — sunrise along one edge, sunset along the other. On the Moon it creeps across the surface at about 12.2 degrees a day, and observers watch it because low light throws the longest shadows.
Selenographic colongitude: The longitude of the sunrise line on the Moon, advancing about 12.2 degrees a day: 270° at new moon, 0° at first quarter, 90° at full, 180° at last. Effectively the Moon’s time of day, and what tells an observer which craters are lit at a low angle tonight.

Counting the Years

Epoch: The zero point a calendar counts from — the Hijra for the Islamic calendar, the Battle of Yavin for Star Wars, 1 January 1970 for Unix time.
Era: A span of years counted from an epoch or a ruler’s accession — Anno Domini, the Japanese Reiwa, the Fourth Era of Tamriel.
Anno Mundi: “In the year of the world” — years counted from a calculated date of Creation, as in the Hebrew and Byzantine calendars. The same initials serve the Coptic Anno Martyrum and the Ethiopian Amete Mihret, which count from quite different moments.
Ab urbe condita: “From the founding of the City” — the Roman count of years from the traditional founding of Rome in 753 BCE.
Nengō: A Japanese era name proclaimed at an emperor’s accession; years count from 1 within it. The present nengō is Reiwa.
Computus: The medieval science of calendar calculation, developed above all to fix the date of Easter — and the root of the word “computer.”
Julian Day Number: A continuous count of days since 1 January 4713 BCE, used by astronomers to sidestep every calendar’s months and leap rules when measuring intervals.
Old Style / New Style: Labels distinguishing Julian-calendar dates (Old Style) from Gregorian ones (New Style) around the 1582 reform, when the two had diverged by ten days.
Correlation constant: The number of days between a calendar’s epoch and the Julian Day count, used to convert between them. The GMT correlation (584283) anchors the Maya Long Count.
UTC: Coordinated Universal Time — the world’s civil time standard, held within a second of the turning Earth by the occasional leap second. Every clock here is your own zone’s offset from it.
YOLD: The Year of Our Lady of Discord — the era of the Discordian calendar, counted from 1166 BCE, which makes our 2026 the year 3192 YOLD.
Leap second: A second inserted into UTC to keep civil time in step with a very slightly slowing Earth. Atomic time (TAI) takes none, which is why it now runs 37 seconds ahead.

Named Reckonings

Kalends, Nones & Ides: The three fixed points of the Roman month — the Kalends (1st), the Nones (5th or 7th) and the Ides (13th or 15th) — toward which every other day was counted backward.
Tzolkʼin: The 260-day Mesoamerican sacred round, pairing thirteen numbers with twenty day-signs; still consulted for divination.
Haabʼ: The 365-day Maya civil year of eighteen twenty-day months plus five unlucky days, the Uayeb.
Calendar Round: The 52-year cycle after which a given Tzolkʼin and Haabʼ pairing recurs — the Mesoamerican “century.”
Long Count: The Maya place-value count of days from a mythic creation date in 3114 BCE; its thirteenth bʼakʼtun rolled over in 2012.
Nawal: A day-sign of the Maya Tzolkʼin, held in K’iche’ tradition to shape the character of those born under it.
Wuku: One of the thirty seven-day weeks of the Balinese Pawukon, whose ten concurrent cycles produce its 210-day round.
Veintena: A twenty-day “month” of the Aztec Xiuhpōhualli; eighteen of them plus five nemontemi make the 365-day year.
Mars Sol Date: A continuous count of sols from 29 December 1873, with no months or years — Mars’s equivalent of the Julian Day, and used the same way, for measuring intervals.
Coordinated Mars Time: Mean solar time on the Mars prime meridian, written MTC. The Martian answer to UTC, and the reference every local Mars time is offset from.
Suriyayart: The traditional arithmetic behind the Thai lunisolar calendar, inherited from Indian astronomy and worked entirely in whole numbers. From the elapsed days of the era it derives the tithi, the position of the moon’s apogee and the leap indicators, and from those the shape of the year. Being arithmetic rather than observational, it can open a month a day away from the true new moon — and the observances follow the arithmetic, not the sky.
Uposatha: The Buddhist observance days, falling on the new and full moon of each lunar month — and in stricter practice the quarter moons as well. They are the reason Theravada countries keep a lunisolar calendar alongside a civil one: the full-moon uposatha of particular months carries Vesak, Asalha Bucha and Magha Bucha.
Adhikamasa: The intercalary month of the Thai lunisolar calendar — a second month 8, added in some years to hold the lunar months against the solar year. In such a year the observances move with it: Visakha Bucha shifts to month 7, and Asalha Bucha to the second month 8. Its companion, the adhikavara, adds a single day to month 7 instead.
Sexagenary cycle: The sixty-year Chinese cycle formed by pairing ten heavenly stems with twelve earthly branches — the branches being the zodiac animals.
Octeday: The eighth day of the Discworld week — Terry Pratchett’s conceit that the Disc runs on an eight-day week.