A month is an amount of time used with calendars. It is about 1/12th of a year. It is a little longer than a natural synodic orbital period of the moon, ca. 29.5 days. Lunar calendars use the natural synodic month. For some purposes astronomers use the sidereal month of 27.3 days.

Other calendars, called "solar calendars", ignore the moon, and depend entirely on the Sun — for example, the Gregorian calendar of 365 or 366 days (12 months) used in much of the world today.