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Almost Decimal Calendar

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In the Almost Decimal Calendar, the year is divided into 5 quinters (alias quintals) or into 10 longths (from “long months”), each of which contains a whole number of weeks in a pattern of 10:11 for the former and 5:5:6:5 for the latter, i.e. longths 3 and 7 last a week longer than the others. Each quinter is split into 2 longths and a year has 10 longths. The leap week is appended to the last quinter and longth when it occurs.

The ISO-like date format is ±CCYY-Q-W-D or ±CCYY-LL-W-D and ±CCYY-Q-L-W-D. The calendar is compatible with that standard as far as possible, e.g. weeks begin on Monday and the year number is always the same as for ISO week dates. Today is 2025-5-9-4 with quinters, 2025-10-4-4 with longths, and 2025-5-1-4-4 with both.

The 1st and 3rd quinter are always short quinters of 10 weeks. In a normal long quinter (i.e. the 2nd and 4th) of 11 weeks, the first longth is 6 weeks long (6:5), but in leap years, the second longth of the last, 5th quinter is a week longer than the first (5:6). This way, longth 5 ends after week 26 and aligns nicely at the half-year mark with most other leap-week calendars.

Relation of the Almost Decimal Calendar to other leap week calendars
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 Summer break Michaelmas trimester
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