A calendar defines important characteristics of the year in a way that is appropriate to the type of analysis you want to perform. By default, the calendar is the standard Gregorian calendar. However, the standard Gregorian calendar is not always appropriate. For example, if your company's financial year begins on June 1 instead of January 1, you could define the first month of the year as June, and the "month of year" time attribute would have June as month 1, July as month 2, and so on, as opposed to January as month 1, February as month 2, and so on.