A calendar is basically a device to identify certain dates and times, with the objective of reminding the user of important days and upcoming events. It may be produced through calendar printing or integrated electronically to computer systems such as the ones found in desktops, PDAs and mobile phones.

Generally, a calendar system just counts days starting from a particular reference day. Others have one or more larger units of time, and may contain one or two levels of cycles. Examples of systems of calendars integrating a year-month-day cycle are the Gregorian calendar, the Islamic calendar, and the Hebrew calendar.

In addition, the cycles are matched with certain periodic events that mark the start of each calendar year: the Lunar calendar is matched with the motion of the moon; on the other hand, a Solar calendar is based on the motion of the Sun. Still others are synchronized with that of Venus, as evidenced from ancient Egyptian calendars.

It is very common to find calendars with two or more types of cycles to mark the start of each year. A lunisolar calendar is one actual example that combines both the motions of the moon and the sun for the cycle. It tracks both the moon phase and the seasons. Examples of these systems are the Chinese calendar, Hebrew calendar, Hindu calendar, and those calendars used in ancient times.

The Solar calendar is based on the dates of each solar day- the period between sunrise and sunset followed by a period of night. Variations in the interval between two successive events (such as two sunsets) are used during the year. It may also be averaged into a mean solar day.

On the other hand, Lunar calendars use each lunar phase cycle to identify the days in the calendar. However, a “drift" is usually expected with every season. In order to correct these drifts, an extra month is added so that the calendar would again be realigned with the seasons. This may be seen in lunisolar calendars, an example of which is the Jewish calendar that has 19 year-cycles. Believed to be invented by the Cro-Magnon around 32,000 BC, the lunar calendars are considered to be the oldest ever invented. The only pure lunar calendar used today is the Islamic calendar.

One of the most popular calendars of today is the Gregorian calendar. Used by almost everyone, including China and India, it is considered as the most effective calendar based on international standards. But due to its obvious connection with Christianity, other non-Christian countries interchange the Gregorian calendar with their own religious calendars. For instance, Israel uses the Gregorian calendar for their business and day to day activities, while the Hebrew calendar is used to mark their religious celebrations. Similarly, China also applies a different calendar for their traditions and holidays.

Other systems of calendars used all over the world are the Persian calendar, Islamic calendar, Ethiopian calendar, among others.