The Octalendar is a perpetual leap week calendar design that divides the year into 8 months of exactly 6 or 7 weeks, alternating 7:6. That means, every odd month has 49 days and every even month has 42 days, except for the last, eighth one in leap years when it has 49 days instead. The date format is ISO-like ±YYYY-M-W-D. All usual date conventions comply with ISO 8601, i.e. weeks start on Monday and the leap week depends on the Gregorian leap rule and year count. Cal7 also has months with 7 weeks, but alternates them with ones of 8 weeks (8:7), totalling in just 7 months.