Calendar units are essential to the proper management of a project. They are the time measurements that are used to plan the entirety of a projects schedule events in a logical manner. Calendar units will typically refer to the smallest possible measurement of time that can be appropriately used in reference to a particular project or a particular activity scheduled within the project as a whole. Possible calendar units are all standard time measurements, such as minutes, hours, days, weeks, months, and, in some long term projects, years. When planning a project, traditionally it is important that the calendar unit represent the same fixed quantity of time throughout the entire planning process of that project, however, it is possible to assign different calendar units to specific schedule activities within the project, particularly if they are going to be completed in a significantly shorter time frame than the project as a whole.
This term is defined in the 3rd and the 4th edition of the PMBOK.