The following is a concise, high-level list of the key subdivisions of the lunar and solar calendars.

  1. Nearly all calendar systems group consecutive days into "months" and also into "years".
  2. In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities.
  3. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.
  4. Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years.
  5. This may be handled, for example, by adding an extra day (29 February) in leap years.
  6. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation.
  7. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.
  8. Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years.
  9. Many cultures use different baselines for their calendars' starting years.
  10. For example, the year in Japan is based on the reign of the current emperor--2006 would be Year 18 of the Emperor Akihito.
  11. In addition to the Christian calendar, the United States also refers to the number of years since American independence in some official documents (i.e., 2006 would be the Year 231--since Year 1 was 1776, when America declared independence).