Calendars are tools that can be used to measure time and record events. They can also be used to plan future events. Cultures all around the world and throughout history have invented a variety of calendars to mark time. Official time is kept here, in the United States, based on the natural cycles of atomic clocks. Our official calendar is also based on the cyclical movements of objects in the sky, as are the majority of calendars kept by other cultures, past and present, including the Maya.