Week-based and Gregorian calendars

Let us first zoom in to what makes a week-based calendar fundamentally different from the Gregorian calendar, and what varieties exist in week-based calendars. We will choose one variety to work with in the remainder of this chapter.

What is a week-based calendar?

While in both week-based and Gregorian calendars, the day is the smallest unit, there is a huge difference in how days are grouped into larger units. While the Gregorian calendar knows about weeks, a week is not a proper hierarchical level in the calendar: most months are about 4½ weeks long, except for February (and not even in leap years). Even February usually starts somewhere in the middle of a week.

A week-based calendar uses the week as a proper hierarchical level from which all higher levels are defined. A week-based calendar does not have a month; instead, weeks are grouped into periods.

The advantage of a week-based calendar is that the end date of a period...