A list of the time intervals during which activities can be worked and / or resources used. Typical data includes working days/non-working days, start and finish times for work-shifts, weekends, holiday periods and extra workdays. Each activity and/or resource will have a calendar attached to it.

A project can contain many calendars, each with different working and non-working periods.

Where a resource may have more than one calendar ie Brickwork on the outside structure using the wet weather calendar and inside using the dry work calendar thenit may be possible to apply the "use calendar as task" button in the resource modelling interface.