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• Gregorian calendar - solar calendar introduced in 1582 by Gregory XIII pope. The base of this calendar is Julian calendar, which was used before. Made changes fixed delay related to tropical year. One year in Gregorian calendar has 365 days (or 366 for lap years) divided into 12 months. Name of the months were inherited from Julian calendar directly.
  
• UNIX timestamp - number of seconds from 1 January 1970 UTC. This date is known as begin of Unix epoch. This format is widely used to store time (and date) in UNIX family operating systems.
  
• Julian day - number of days from 1 January 4713 B.C., counted using Julian calendar. This date was determined by Joseph Scaliger in 1583 year. Julian days are widely used in astronomical calculations.
  
• Modified Julian day - Julian day with start date moved to 17 November 1858 year. Modified Julian day can be simply computed from Julian day by subtracting 2400000,5 days from it.
  
• Julian calendar - solar calendar invented by greck astronom Sosygenesa under Julius Cezar order. Introduced as official Rome calendar in 45 B.C. This calendar was widely used in whole Europe until Gregory calendar was invented in XVI century. Today, used for religious purposes - it's a base for liturgical year in the Orthodox Church.
  
• Hebrew calendar - oryginally lunar (solar-lunar today) calendar, used by Semitic tribes. Most recent form was invented in 359 year by Sanhedryn under Hillela II order. One year in Hebrew calendar has 354, 355 or 356 days (383, 384 or 385 in lap years) divided into 12 or 13 months. Calendar begins at 7 October 3761 B.C. This date is known day creation of the world according to Jewish beliefs.
  
• Islamic calendar - lunar calendar used mainly by Muslims for religious purposes. Calendar begins at 622 A.D. One year in Islamic calendar is divided into 12 months containing 29 or 30 days.
  
• Persian calendar – calendar invented in Persia in XI century. Today, used in Afghanistan and Iran. Introduced in 1079 A.D. Persian calendar was developed among others by Omar Chajjama under Malik-Shah I sultan order. One year is divided into 12 months containings 30 or 31 days, excepting Esfand month, which has 29 days in non-leap years.
  
• Kurdish calendar - solar calendar related with Persian calendar. It begins at 612 B.C.
  
• Indian national calendar - calendar officialy introduced in India at 22 March 1957 A.D. Developed 5 year before by Calendar Reform Committee under India government commision. Tere were many alternative calendars used in India, before Civil Indian calendar was introduced. One year in this calendar has 365 days (or 366 in leap years) divided into 12 months. Calendar begins at 78 A.D. to commemorate the victory over Saka.
  
• Mayan long count - historical calendar used by Mayan civilization beetwen III and IX century. The date is five digit number recorded in twenty system, which means the number of days from 11 August 3113 B.C. Digits have names (from most significant): baktuns (144000 days weight), katuns (7200 days weight), tuns (360 days weight), uinals (20 days weight) and kins (1 day weight).
  
• Excel serial day (PC, 1900) - time representation format used internally in Microsoft Excel software on PC computers (Windows). It's a number of days from 1 January 1900.
  
• Excel serial day (Macintosh, 1904) - variation of Excel serial day with begin date moved to 1 January 1904. This format is used on Macintosh computers, because early version of MacOS systems had no support for dates before 1904 year.
  

• alternative calendars calculator
  
• wikipedia: Gregorian calendar
  
• UNIX timestamp calculator
  
• Julian days calculator
  
• More about Julian daysr
  
• more about Julian calendar
  
• Hebrew (Jewish) calendar online
  
• more about Islamic calendar
  
• Persian calendar online
  
• Persian to Gregorian (and vice versa) converter
  
• more about Indian civil calendar
  
• more about Mayan calendar
  
• Microsoft Excel serial day (1900 and 1904)