The Shift Calendar is not a calendar (proposal) by itself, but a supplementary concept that is compatible with any calendar that has arranged in 3 months of or 3 months of and 1 extra day outside the month cycle. It works well with , but probably better with leap-day calendars. The idea is to define several shift plans and work week independent of the actual 7-day week, but fixed to the calendar, cf. market weeks and Indonesian pawukon. Normal 7-day weeks and 13-day cycles are the only ones which complete the 91 days of a quarter without holiday.