The ISO Uncia Calendar is a proposal to incorporate the week numbering rules of ISO 8601 into an auxiliary calendar format that can be put to use at any time by anyone, because it serves to supplement the existing Gregorian calendar and ISO 8601 rules without changing them.
The year is first divided into four equal quarters of 91 days/13 weeks (the last quarter will have 98 days in leap years). Each quarter begins on Monday of W01, W14, W27, and W40 of the ISO week numbering system. The leap week will be added whenever ISO 8601 specifies a 53rd week.
Each quarter is further divided into 3 divisions which are called uncia (Latin for twelfth). They are called such so that they will not be confused with the months of the Gregorian calendar, which they will not replace, and with which they will not precisely correspond.
Each uncia will have 30 days, except the third, which will have 31. (The very last, or 12th, will have 38 days in a leap year.) The longer unciaeis at the end of each quarter. In this way, the first 90 days of the quarter can be easily divided into useful day-groups of 3, 5, 6, 9, 10, 15, 30 or 45 days, which can conveniently fit into the 90-day scheme of 3 unciae, minus the last day (or last week of the leap week year). Of course, with the 91st day, the quarter can be divided into 13 7-day weeks.
The unciae will always begin on Monday, Wednesday or Friday, depending upon whether the uncia is respectively the 1st, 2nd or 3rd unica of a quarter.
The unciae will simply be identified by ordinal numbering (1st thru 12th). As an option, for those who prefer names, they can be called by ordinal numbers in Latin:
They can be numbered in a format similar to the ISO day numbering scheme as ccyy-Uuu-dd, wherein Uuu – with initial literal capital ‘U’ – represents the uncia, such that U03 would represent the 3rd uncia, or Tertia, and dd would represent the day number of that particular uncia.
Every day of each uncia will always correspond to a particular week-day number in the ISO week numbering scheme. For instance, U05-19 (19th day of Quinta), will always correspond to W20-7, a Sunday. U10-10 (10th day of Decima), will correspond to W41-3, a Wednesday. And so on, for every day of the ISO week numbering system. There will never be a difference, as long as the ISO rules are not changed.
Given that the ISO-Uncia Calendar is an auxiliary, or supplemental calendar, rather than a proposal to replace the existing Gregorian calendar, or change any current ISO standard, or modify the sequence of weekdays, it does not require any authorization from any legislature, standards setting body, or religious denomination. Anyone who finds the Calendar useful, is free to implement it anytime that they want in their own way.
A couple of added bonuses of the ISO Uncia calendar:
- The format is very similar to the ancient Enoch calendar, and to several proposed leap week calendars.
- There are no Friday the 13th's!