Weekdate is a leap week calendar that is compatible with ISO 8601 except for the leap rule and notation. It was designed by Rick McCarty.
Notation[]
In English, the date is given as literal day of the week (dow), followed by the ordinal week number and the year, separated by a comma. The numeric format uses the forward slash (solidus) as a separator and has the same ascending order of items, the dow digit is preceded by a letter ‘d’, the week number has no prefix (unlike in ISO week dates).
Leap rule[]
The week date calendar shares a leap cycle with the Gregorian calendar, but since it has to spread leap weeks – not days – across a span of 400 years, the rule differs.
Every fifth year, i.e. each having its year number end with 5 or 0, has 53 weeks, with 9 exception per cycle. Years matching 400·n + [035, 085, 125, 170, 210, 255, 300, 345, 390], where n is 5 currently, have just 52 weeks. That is, when starting with year 0, the 8th, 18th, 26th, 35th, 43rd, 52nd, 61st, 70th and 89th possible leap weeks are left out.
This results in larger deviation from the Gregorian start of the year (1 January) than with ISO rules, but has minimum deviation for any rule, where every fifth year with exceptions has 53 weeks. The new year ranges from 26 December through 6 January, over 12 days.
Properties[]
Since this calendar proposal does not alter months in any way, because it doesn’t use them, it could be used in parallel with the Gregorian calendar. The author states that since there is no global authority for extensive calendar reforms, such “Trojan horses” are the only realistic way to actually reform the calendar.
Deciding whether a year has a 53rd week is a bit easier than with ISO 8601.