The Tetraeteric calendar is a lunar (partially lunisolar) calendar invented by Hellerick in 2000. The months are grouped into four-year terms tetraeterides (singular tetraeteris). One tetraeteris consists of 49 lunar months, or 1447 days without exception.
Months of tetraeteris are grouped into four solar cycles ("years") of twelve months similar to lunar years, with addition of yet one embolismic month that does not belong to any solar cycle.
All months have either 30 or 29 days. The months of tetraeteris are grouped into three periods (similar to yerms in Yerm Lunar Calendar), with first two being 17 months long and the third one being 15 months long. All periods start and end with 30-day months, and alternation of 30- and 29-day months in between.
The months are named after the months of the Attic calendar. The months names are repeated within every 12-month solar cycle: Elapheboly, Munichy, Thargely, Scirophory, Hecatombey, Metageitny, Bedromy, Pyanepsy, Memactery, Poseidey, Gamely, Anthestery. The last additional month has name Emboly.
The table below shows the length of the months:
Thus every tetraeteris has four Elaphebolies, four Munichies etc. To distinguish them you have count them: First Elapheboly, Second Elapheboly, Third Elapheboly, Fourth Elapheboly. So, the first month of the tetraeteris is the First Elapheboly; the First Anthestery is followed by the Second Elapheboly; and the second to last month of the tetraeteris is the Fourth Anthestery. The last month, Emboly, is the only month with such name and doesn't have to be counted. Informally the first solar cycle is known as "the first months", the second as "the second months" etc.
The months of the tetraeteris effectively form three yerms; the first and second have 17 months each and the third has 15 months. They begin at First Eleapheboly, Second Metageitny and Third Gamely and may be useful in working out the number of days in a month.
The date is expressed in the next style: 21st day of the First Bedromy of the 236th tetraeteris. Its "ISO" format is TTT-S/MM-DD: 236-1/07-21, in which Emboly is treated as the 13th month of the fourth solar cycle. But a more common format is D-Mmm.S-TTT in which month are expressed with three-letter abbreviations, and the solar cycle for Emboly is not marked: 21-Bed.1-236.
The tetraeteric era begins on September 22nd, 1066 CE of the Julian Calendar, or September 28th, 1066 CE of the Proleptic Gregorian calendar. It was the date of a new moon and a solar eclipse, three days before the Battle of Stamford Bridge and 22 days before the Battle of Hastings, which marked the crucial moment in the history of England. The first day of the tetraeteric era is the first day of the First Elapheboly of the first tetraeteris (001-1/01-01).
238th tetraeteris started on 2005-09-04 CE. 239th tetraeteris will start on 2009-08-21 CE.
- All the tetraeterides are the same, i.e. there are no leap tetraeterides.
- Its average month length is 29.53061 days, which is more accurate than in lunar tabular Islamic calendar.
- Its average year length is 361.75 days, 3.5 days short, which is much more close to an actual value than in common lunar calendars.
- In contrast to Yerm Lunar Calendar it preserves the idea of year.
- It's difficult to remember how long months are without looking at the table. You have to remember that 30-day months are: odd-numbered months 1st Elapheboly through 2nd Hecatomby, even-numbered months 2nd Metageitny through 3rd Poseidey, and odd numbered months 3rd Poseidey through Emboly; while the rest are 29 days long.
- It's not as accurate as most lunisolar calendars. In 52 years it shifts half year backwards, which makes summer holidays fall on winter. On the other hand in common lunar calendars it happens in 16 years only.