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.

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.
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.
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.

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.
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.