A base calendar is a calendar that is used by another calendar to provide working days, hours, and events. For example, an Organization calendar is used by Project calendars as a base calendar to give the Project calendars the Organization's working hours and events (the Project calendar "inherits" the base calendar working hours and events). This way you can create a Project schedule that fits within the working hours set by the Organization.
For example, imagine an Organization calendar called Full-time that has working hours Mon-Fri, 9-5. Now imagine MyProject's calendar specifies calendar Full-time as its base calendar. MyProject's Calendar inherits the working hours from Full-time Calendar; if Full-time Calendar had any events, such has holidays; those events would also be reflected in MyProject's Calendar. You can then refine MyProject's Calendar to meet the needs of the project, for example by scheduling recurring meetings that are specific to the project.
Organization calendars can be base calendars for Project and Resource calendars. While Tasks can use Organization or Project calendars (see Tasks Base Calendar field), they cannot override them.
Making changes to a base calendar
Once a calendar is in use as a base calendar, you cannot change the base calendar's working hours. You can add events to a base calendar. For example, if the base calendar's working hours for Friday are 9-5, you could add an meeting for Friday 3-5. An alert will be sent to the project owner informing him or her of the change to the base calendar, and the Requires Rescheduling attribute on the project will be set to Yes. At this point, the project owner can reschedule the project.
Once a calendar is in use as a base calendar, you cannot delete it. You can, however, make a base calendar inactive. This means that any calendar currently using the base calendar still uses the calendar as a base calendar, but the base calendar will not be available for use with any new calendars.
Note: You cannot make the default Organization calendar inactive.
While changes to events on a base calendar will affect existing project, and resource calendars, these changes will not override any local customizations made by the resource to his/her resource calendar or by the project owner to the project calendar.
Any changes made to a project or resource calendar will not affect its base calendar.
Assigning base calendars
If you are a supervisor, you can assign base calendars to resources. For example, you might have a team of full-time resources and a team of part-time resources. You can assign the Full-time Organization calendar to full-time resources, and the Part-time Organization calendar to the part-time resources. Similarly, project owners can choose the appropriate Organization calendar to use as a base calendar for each of project.
Calendar effective dates
Project and Resource calendars can use multiple base calendars, each with unique effective dates. For example, a resource might use the Full-time Organization calendar as a base calendar until Dec 31 2011, and then use the Part-time Organization calendar from Jan 1 2012 onward. The change to the resource's capacity will take effect starting Jan 1 2012.
See Using Effective Dates for Base Calendars for more information.