Different civilizations used their own calendars (ie. systems of measuring time). The examples are:
- Julian calendar (used in ancient-rome),
- Gregorian calendar (commonly in use since 16th-century),
- Chinese calendar,
- Hebrew calendar,
- Mayan calendar,
- Islamic calendar,
- Hindu calendar,
- French revolutionary calendar,
- Unix (calculated as seconds passed since 1970-01-01)
As measuring time is one of oldest social human activity, every civilization created its own calendar. The difference come from taking different bases, however, they are mostly based on astronomical events.
Some of them are somehow related to astronomical year, ie. number of days that pass for the whole movement of Earth around the Sun (about 365,24 days). This number is not integer and for many years it was difficult to perform exact calculations. Some calendars set as their base the lunar month, ie. time that passes during the whole movement of Moon around Earth (about 28 days).
The beginning of the calendar (considered as the 1st year) is set to important historical event (eg. foundation of Rome in Julian calendar, French revolution etc.), religious event (creation of World by God in Hebrew calendar, birth of Jesus Christ in Christian calendar etc) or other.
The calendar now in use is Gregorian calendar, divided in two eras, with modern era (abbreviated AD as Latin Anno Domini) starting on estimated date of birth of Jesus Christ (Jesus of Nasareth), and former era, abbreviated BC (before Christ). This calendar probably fits best to astronomical year. There are 365 days in the year, and each year that is dividable by 4 has 366 days (a leap year). The years that are dividable by 100 do have 365 days, the years that are dividable by 400 (the last one was 2000 AD) do have 366 days.
The year is divided by 12 months, the same as in Julian calendar. Months have different number of days. Each day consists of 24 equal hours, each hour consists of 60 minutes, each minute consists of 60 seconds. Seconds are divided in decimal systems, a milisecond (1/1000 s) is commonly used. Each seven following days form a week.
In most calendars, periods of time (like months, days of week etc.) have their own names.
Some of days are considered feasts and some are usual days.
The standard of writing time (the date) is ISO 8601, however, many people use common methods.
The common use of the word "calendar" is for printed or other way stored and organized drawing of the time, eg. during a specific year.
Questions related to systems of measuring (abandoned or still in use) time should be tagged with calendar. This concerns both the dates and time of day. Also, this tag could be used to mark questions related to methods of writing time.
The questions can be also related to history of determining moveable, reoccurring events, where date of Christian Easter is one of best examples.