• Any device for keeping track of the passage of dates. Most calendars are astronomical in basis, depending on the Sun and Moon. But they all suffer from the problem that the day, month and year are slightly variable and do not represent simple fractions or multiples of each other. Much ingenuity has been exercised on this problem over many millennia, giving rise, e.g., to the Gregorian calendar now in use in most of the world, which adds a leap day when the year number is divisible by four, except in a century year, which has a leap day only if divisible by 400. This system is accurate to one day in over 3000 years. Increased accuracy in measurement brings new complications. Time is now measured by atomic clocks, which are more regular than the Earth’s rotation, which is gradually slowing. This necessitates the insertion of ‘leap seconds.’


  • noun a list of dates, especially a list of dates of new share issues


  • Defined work periods and holidays that determine when project activities may be scheduled. Multiple calendars may be used for different activities, which allows for more accurate modeling of the project work plat (e.g., 5-day work week calendar vs. 7-day work week).

Information & Library Science

  • noun a printed table or chart which shows the days, weeks and months of the year


  • noun a book or set of sheets of paper showing all the days and months in a year


  • noun a list of events with the dates on which they will take place
  • noun a list of Bills for consideration by committees of the House of Representatives or the Senate


  • noun a book or set of sheets of paper showing the days and months in a year, often illustrated with a series of pictures

Origin & History of “calendar”

English acquired calendar via Anglo-Norman calender and Old French calendier from Latin calendārium, which was a ‘moneylender’s account book’. It got its name from the calends (Latin calendae), the first day of the Roman month, when debts fell due. Latin calendae in turn came from a base *kal- ‘call, proclaim’, the underlying notion being that in ancient Rome the order of days was publicly announced at the beginning of the month.

The calendula (19th c.), a plant of the daisy family, gets its name from Latin calendae, perhaps owing to its once having been used for curing menstrual disorders. Calender ‘press cloth or paper between rollers’ (15th c.), however, has no connection with calendar; it probably comes from Greek kúlindros ‘roller’, source of English cylinder.