Calendar

This is done by giving names to periods of time, typically days, weeks, months and years.

Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun or the moon.

[9] Latin calendarium meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month).

[10] The Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century (the spelling calendar is early modern).

[10] The course of the Sun and the Moon are the most salient regularly recurring natural events useful for timekeeping, and in pre-modern societies around the world lunation and the year were most commonly used as time units.

[13] According to Yukio Ohashi, the Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures.

[18] Calendars in antiquity were lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years.

This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar.

[19] His "Julian" calendar was no longer dependent on the observation of the new moon, but followed an algorithm of introducing a leap day every four years.

This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.

Virtually the only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller.

[25][26] The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day.

However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.

Alexander Marshack, in a controversial reading,[32] believed that marks on a bone baton (c. 25,000 BC) represented a lunar calendar.

Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar.

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.

Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation.

The Gregorian calendar is the de facto international standard and is used almost everywhere in the world for civil purposes.

[37] There is a lunar aspect which approximates the position of the moon during the year, and is used in the calculation of the date of Easter.

[41] The most important use of pre-modern calendars is keeping track of the liturgical year and the observation of religious feast days.

While the Gregorian calendar is itself historically motivated to the calculation of the Easter date, it is now in worldwide secular use as the de facto standard.

Some Christian calendars do not include Ordinary Time and every day falls into a denominated season.

With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately every 33 Islamic years.

The Hebrew calendar is used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays) and can be used business dealings (such as for the dating of cheques).

The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and social purposes.

Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates.

In a paper calendar, one or two sheets can show a single day, a week, a month, or a year.

If a sheet is for multiple days it shows a conversion table to convert from weekday to date and back.

With a special pointing device, or by crossing out past days, it may indicate the current date and weekday.

The software may be a local package designed for individual use (e.g., Lightning extension for Mozilla Thunderbird, Microsoft Outlook without Exchange Server, or Windows Calendar) or maybe a networked package that allows for the sharing of information between users (e.g., Mozilla Sunbird, Windows Live Calendar, Google Calendar, or Microsoft Outlook with Exchange Server).