In the Double-Month Calendar or, more precisely, Double-Month Week Calendar (DMWC), the year is divided into 6 sesters, alias sextals, bimesters or double-months, each of which contains a whole number of weeks in a pattern of 9:9:8, i.e. there are either 56 or 63 days in a bim.
Notation[]
The ISO-like date format is ±YYYY-S-W-D
. The calendar is compatible with that standard as far as possible, e.g. weeks begin on Monday.
For example, today is 2024-2-6-5, the same as 2024-W15-5 or, less obviously, 2024-04-12.
If disambiguation with similar schemes is required, e.g. from Octalendar’s ±YYYY-M-W-D
with 6–7-week months, the sester number may be prefixed by a single-letter marker B
for bimonth, e.g. 2024-B2-6-5.
The more intuitive choice S
was avoided because it may more readily be associated with a quarter-year season, as in the International Calendar, or a 6-month, half-year semester.
The hyphen separators are optional then, 2024B265. With just three digits, this notation is more efficient than most commonly used ones that use at least 4 digits.
Leap week[]
The leap week is appended to the last sextal at the end of the year; that means only the third sester has just 8 weeks then, all the others have 9 weeks. This avoids a 10-week bim and is the reason for choosing the pattern 9:9:8 over 9:8:9 or 8:9:9.
This happens in exactly the same years that have a W53 by ISO rules. Unfortunately this is a very complex and seemingly irregular pattern with no simple algorithm.
Subdivision variants[]
Each sextal can be matched by two months of 4 or 5 weeks exactly. The possible patterns for half a year, i.e. three consecutive sextals, are
- 5:4 : 5:4 : 4:4,
- 5:4 : 4:5 : 4:4,
- 4:5 : 5:4 : 4:4 and
- 4:5 : 4:5 : 4:4.
The last one could be dubbed “4:5*” and is harmonic for the second half of the leap year. The second one, on the other hand, is equivalent to the 5:4:4 pattern, ensuring that there are never 3 consecutive short months with just 4 weeks each. Both of these preferable patterns align well with 13-week quarters. The “symmetric” week-month pattern 4:5:4 would yield a different pattern for sextals, 9:8:9, and 4:4:5 would result in another, 8:9:9.
Week | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
13mon | Janon | Febron | Marchon | Apron | Mayon | Junion | Extron | Julon | Augon | Septon | Octon | Novon | Decon † | ||||||||||||||||||||||||||||||||||||||||
5:4:4 | Janaire | Febraire | Marchose | Aprilose | Mayose | Junal | Julial | Augial | Septidor | Octidor | Novidor | Decaire † | |||||||||||||||||||||||||||||||||||||||||
4:5:4 | Janose | Febrose | Aprial | Mayal | Julidor | Augidor | Octaire | Novaire | |||||||||||||||||||||||||||||||||||||||||||||
4:4:5 | Febral | Marchal | Maydor | Junidor | Augaire | Septaire | Novose | Decose † | |||||||||||||||||||||||||||||||||||||||||||||
Quarter | quart 1 | quart 2 | quart 3 | quart 4 † | |||||||||||||||||||||||||||||||||||||||||||||||||
7:6 | Bol | Sar | Tan | Lit | Nas | Mon | Sam | Yul † | |||||||||||||||||||||||||||||||||||||||||||||
4:5* | Janose | Febrose | Marchose | Aprilose | Mayose | Junal | Julidor | Augidor | Septidor | Octidor | Novidor | Decaire † | |||||||||||||||||||||||||||||||||||||||||
9:9:8 | sester 1 | sester 2 | sester 3 | sester 4 | sester 5 | sester 6 † | |||||||||||||||||||||||||||||||||||||||||||||||
5:5:6:5 | Primoth | Secoth | Tertoth | Quaroth | Quintoth | Sixtoth | Septoth | Octoth | Novoth | Decoth † | |||||||||||||||||||||||||||||||||||||||||||
10:11 | quinter 1 | quinter 2 | quinter 3 | quinter 4 | quinter 5 † | ||||||||||||||||||||||||||||||||||||||||||||||||
8:7 | Monmonth | Tuesmonth | Wednesmonth | Thursmonth | Frimonth | Saturmonth | Sunmonth † | ||||||||||||||||||||||||||||||||||||||||||||||
Terms | Spring semester | Summer break | Fall semester | ||||||||||||||||||||||||||||||||||||||||||||||||||
Winter semester | Spring break | Summer semester | Fall break | Winter semester | |||||||||||||||||||||||||||||||||||||||||||||||||
Epiphany/Hilary trimester | Trinity trimester | Michaelmas trimester |
See also[]
- Newcal and the 6*6*10 regular calendar use a 61-day double-month, paired with 5-, 6- or 10-day weeks.