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Jottings #6: Calendars

This article originally appeared in the September/October 2018 issue.

Calendars are such a routine part of life that we don’t normally think about how they work. However, when you’re doing worldbuilding and culture building, they can become an important part of what establishes the ‘feel’ of your work.

This article discusses/describes the various types of calendars. It does not discuss converting between calendars, nor does it actually define any particular calendar for a particular world.

The phrase “metaphysical significance” is used as shorthand for indicating that something has significance to a culture for reasons other than grounded in real, measurable phenomena. As an example, the seven-day week of most modern cultures is ultimately based on no more than the creation legend in scripture; thus, ‘seven’ has metaphysical significance. On the other hand, the year being 365 days is based on the measurable time over which the stars progress around the sky to return to the same position; this is not metaphysical significance.

“Day” and “Year” refer to both the calendar periods and the rotational and revolutional periods (respectively) of the world; where it is necessary to distinguish, a phrase such as “solar year”, “calendar year”, etc., will be used.

“Week” and “month” are used to refer to multi-day periods on the calendar; generally, the “week” is ten days or less; the “month” is 20 days or more; periods in the gap are defined on a case-by-case basis. Where the orbital period of a natural satellite is referenced, it will be called a lunation.

“Month-date dating” refers to uniquely identifying a date within the year using subdivisions of the year (“months”), and the day within the month. This is the “conventional” model for most calendars in current use. “Month-date dating” may place the month and date in either order (e.g., “1 January” or “January 1”); the important feature is that both are required. In Traveller, the Solomani, Zhodani, and K’kree calendars use this method.

“Serial dating” refers to uniquely identifying a date within the year by simply counting days from the beginning of the year. In Traveller, the Imperial, Vilani, Aslan, and Hiver calendars use this method. The Mayan “Long Count” can also be considered serial dating (on a non-annular calendar), as can the “Julian Day” system used by astronomers.

Types of Calendar

Fiat Calendars

Fiat calendars are calendars that are established by law or edict, and do not have any necessary connection to astronomical phenomena or metaphysical significance on the world(s) they apply to. The Imperial calendar may be a fixed or solar calendar for Sylea, but is a fiat calendar for any other world in the Imperium. Fiat calendars may have any of the structures discussed in this article.

Fixed Calendars

Fixed calendars do not change based on astronomical phenomena, and do not have any necessary connection with them. Examples of fixed calendars are Traveller’s Imperial, Vilani, and K’kree calendars, and the Mayan Haab and Tzolkin calendars. If based on a solar year, fixed calendars still do not intercalate (even if a true solar calendar would). Most known fixed calendars were intended as solar calendars, but were defined under conditions where it was not clear that intercalation would be needed, or was considered undesirable or inappropriate.

Haab-style calendars
are named for the Mayan civil calendar of that name. They divide the year into a number of shorter periods (‘months’, for this article’s purposes) of exactly equal numbers of days. If this leaves a deficit as contrasted with the number of days in the year, the remaining days are left as a period not part of any month (“Uayeb” or “Wayeb” for the Mayan Haab calendar), or, rarely, as a “short month”.

Haab-style calendars use month-date dating. The number of months in such a calendar may, but need not, have metaphysical significance; the number of days in the month is defined by the number of months—though the number of months may be chosen to make the length of the month close to the lunation.
Tzolkin-style calendars,
named for the Mayan religious calendar of that name, are based on two cycles of days, whose numbers are mutually prime. Both cycles increment daily; each day on the calendar is a unique combination of positions in the two cycles, and a year on the calendar consists of a number of days equal to the product of the lengths of the two cycles. An example of how a Tzolkin-style calendar works can be easily illustrated; assume that the two periods are five and three days respectively. The five-day cycle names the days, “Red”, “Yellow”, “Orange”, “Green”, and “Blue”; the three day cycle simply counts the days. A year on such a calendar consists of the dates, in order, Red 1, Orange 2, Yellow 3, Green 1, Blue 2, Red 3, Orange 1, Yellow 2, Green 3, Blue 1, Red 2, Orange 3, Yellow 1, Green 2, Blue 3. The lengths of the two cycles may be chosen for their metaphysical significance, or to be as close to the length of the solar year as possible within the mutual-primality constraint. Either or both cycles may use names instead of numbers, and the naming of the days may place either cycle first (e.g., “1 Red” could be the name of the first day in the previous example).

Solar Calendars

Solar calendars use the Solar Year as their fundamental period. The Calendar Year is as close as possible to the Solar Year, to an integral number of days. Intercalation will generally amount to one day every few years, unless the calendar is modified to accommodate numbers or periods that are significant from a social or religious perspective (which may lead to longer intercalations at different intervals). The Gregorian calendar commonly used today is a solar calendar (and remains one for Terra as the Solomani calendar used in Traveller, though it is a fiat calendar for the rest of the Solomani Confederation), as are the Zhodani and Aslan calendars (for their respective racial homeworlds); many other regional calendars in use today are also solar calendars. A solar calendar can be structured as a Haab-style calendar; in such a case, intercalary periods, when needed, would be included in the Uayeb-equivalent period.

Lunar Calendars

Lunar calendars use the lunation as their fundamental period. The calendar month is as close as possible to the lunation, to an integral number of days. Lunar calendars may be observational, with the new month beginning when the first bit of crescent moon after the new moon is visible, at a specified location, or they may be calculated, with the new month beginning when the new moon would be ‘observed’ at a specified location under ideal conditions, regardless of actual observation. A calendar year is some fixed number of months, approximating the solar year, but without regard for long-term accuracy (no intercalation, no Uayeb-equivalent period, etc.). The Islamic calendar is an observational lunar calendar; Islamic calendars printed in advance for planning purposes are calculated lunar calendars.

Lunisolar/Hybrid Calendars

Lunisolar (or hybrid lunar/solar) calendars use the lunation as the fundamental period, as the lunar calendar does, and may also be observational or calculated. Normally, the calendar year is a fixed number of months, but long-term accuracy is considered, and when the deficit from the fixed number of months accumulates to the point of being equal to or greater than an entire lunation, an additional month for that year will be added to the calendar. The Jewish (often improperly called Hebrew) calendar is a calculated lunisolar calendar; intercalation of an additional month happens in 7 years of each 19. The traditional Chinese calendar is also a lunisolar calendar. The additional month may be added in a fixed location within the calendar, as with the Jewish calendar, or in a varying location as defined by multiple factors, as in the traditional Chinese calendar.

Tracking years

In general, a calendar counts years from a real or stipulated “epoch”. There are several ways of defining the Epoch:

Regnal Epochs:
Dating from the beginning of a monarch's reign. This is common in ancient documents, e.g., “In the sixth year of the reign of Katsandogs…”, and in east Asian pre-modern historical documents; it is still practiced in Japan (other east Asian countries have abolished their monarchies).
(Stipulated) Creation of the World:
Several current calendars use this, the best known being the Jewish calendar; no two seem to use the same date as the epoch date. The Mayan Long Count also stipulates that (corresponding to 11 August 3114 BCE in the proleptic Gregorian calendar) was the creation date of the world.
Stipulated or actual date of a significant event in the life of a significant person:
The Julian and Gregorian calendars use this, dating from the stipulated brit milah (ceremony of circumcision) of Yeshua ben Yosef of Nazareth, held to be the Messiah in Christianity.
Stipulated or actual date of a significant event in the culture:
The Romans used this, dating “Ab urbe condita”, “from the founding of the city [of Rome]”. This is also common in Science Fiction, where a local calendar is often dated “A.L.”, “After Landing”, or in some older SF, “A.E.”, “Atomic Era” dating, usually considering the epoch to be either the first detonation of an atomic weapon, or the activation date of the first atomic pile. The Islamic calendar, dating from the Hejira (flight of Mohammed from Mecca to Medina), and the short-lived French Revolutionary calendar also used this model.

Mesoamerican cultures—most notably the Aztec and Maya—didn’t number their years; instead, they used the Haab and Tzolkin calendars in conjunction. A “calendar round” was the period between two successive occurrences of the same date pairing on the two calendars; this interval was approximately 52 years. Within the 52-year calendar round, every combination of a Haab date and a Tzolkin date was unique. Where greater accuracy was required, the calendar round date would be coupled with the Long Count for the date.

Counting Days

The most common way of counting days within a year or month is to simply increment (generally from 1) throughout the period. The Roman calendar, however, used an inclusive decremental system, counting down toward one of three significant days during the month: Kalends (the first day of the month, sometimes Calends), Ides (one day before the middle of the month), and Nones (eight days before the Ides of the month). In an inclusive decremental system, one counts the day itself as ‘one’, and counts backward to it—so that if the Ides of the month falls on the 14th of the month, the 12th was called “the third day before the Ides of …”. An exclusive decremental system would make the 12th “the second day before …”


Many solar (and lunisolar) calendars have origins in agrarian societies; as a result, certain seasonal events take on an outsized importance—for example, the ancient Egyptians were focussed on the annual flooding of the Nile River; many cultures placed high importance on the solstices and equinoxes. Where a solar year is not comprised of an integral number of days, the calendar year will gradually get out of synchronization with the solar year. To resynchronize the two, one or more days may be added to a calendar year. These days are called intercalary or embolismic days; the process of adding them is called intercalation. The rules for where intercalary days are added to the year will vary depending on culturally-significant factors of many types; while the most common model is to add the additional days at the end of a period, or between periods, there are recorded instances where the days were inserted within and interrupting a period—for example, the intercalation on the Gregorian calendar is sometimes called the bissextile; this is because when the practice was instituted with the Julian calendar, the intercalary day was inserted as a repeat of the sixth day before the kalends of March (see “Counting Days”, above). At one point, when intercalation of the Roman calendar involved adding an additional month, this month was also generally inserted between the sixth and seventh days before the kalends of March.

Wrapping It All Up

The interaction between culture and calendar when you’re doing worldbuilding can be complex, and influences can go in either direction—or both. As mentioned earlier in this article, you needn’t restrict yourself to a single calendar; two (or more!) can exist for various reasons, and can be considered in isolation (as, e.g., an interstellar fiat calendar and a local solar calendar), or can interact in culturally-significant ways (as, e.g., the Haab and Tzolkin). You could have multiple lunar or lunisolar calendars on a world with multiple moons, or multiple solar calendars for a world in a polystellar system. Intercalation can be nothing more than an “accounting entry” (more-or-less how the intercalation of the Gregorian calendar is treated), or it can take on cultural significance (as in Olympiad Day and Double Olympiad Day on the Zhodani calendar). You decide the limits!