Most people in the world work to the calendar known as Common Era but there are, amongst others, Jewish, Muslim and Chinese Calendars also in use. At any moment in time today by the Western Calendar people are living in 2 days at the same time! So when people in Greenwich, England are having their afternoon tea on a Thursday whilst in Greenwich, just outside Sydney, in Australia that are sitting down to breakfast on a Friday!
We all appear to live in the same day by our local calendars at the moment it is midnight at the International Date Line. To avoid this confusion of people living in 2 days at the same time a Universal Day was created by International Convention in 1884 in Washington DC, USA . This Universal Day operates to World Time or Universal Time at Greenwich, England; historically referred to as GMT (Greenwich Mean Time). It begins at midnight GMT (I.E. 12 noon at the International Date Line)
This was developed principally with the introduction of railways (railroads) since prior to that most towns and cities had their own local time. Just like 'planes today fly between time zones then train timetables were confusing and passengers confused.
Today with universal communications systems: aircraft, satellite systems even email there needs to be a single reference point for the World.
That reference point is midnight at the cross-hairs of the Airy telescope in the Old Greenwich Observatory in Greenwich, England. It is also the point of zero longitude; the Prime Meridian of the World also known as the Greenwich Meridian which can be seen on all modern World Maps.
All time and space on planet earth is measured by 2 reference lines. The lines of longitude based on the Greenwich Meridian (0° Longitude) and the Equator (0° Latitude). You can express where you are precisely by your latitude and longitude and in time by Greenwich Mean Time.
Calendar systems are based around major historical events and calendars have continually being altered and corrected and sometimes completely redefined. The calendar base that we use today was not conceived until the year 525 AD. At that time the Roman Calendar was still being used which was based on the founding of the city of Rome on April 22, 753 BC. The new calendar was conceived by a monk called Dionysius Exiguus. He proposed that Christ was born in the year of Rome 753 but most historians agree that it should have been some years earlier. The Venerable Bede, an English monk, through his writings in the 8th century re-chronicled history. Up to that time people had been living, say, in the (Roman) year 1500 suddenly found they were living in 747 AD. (I.E. Don't believe anyone who finds a coin with a year in the first 6 centuries AD (they didn't exist for the people living in that time)!
Changes have happened since Pope Gregory XIII adjusted the calendar in 1582 AD, who eliminated the accumulated error caused by a faulty calculation of the length of a year and avoided its recurrence by restricting century leap years to those divisible by 400. (Yes 2000 IS A LEAP YEAR). This was introduced in Roman Catholic countries and other states only gradually changed from Old Style to New Style; Britain and its colonies didn't adopt the Gregorian calendar for almost two centuries in 1752 AD, when the error amounted to 11 days. The 3 September 1752 AD became 14 September 1752 AD. Up until then England had celebrated beginning of the year on 25 March; after 1752 it was moved to 1 January.
The last millennium would have been celebrated in Greenwich, England on 25 March 1001 AD but it wasn't a thousand years ago because we "lost" 11 days along the way!
The "millennium" before would have been celebrated in Greenwich, England on 25 March 347 AD. Why? Because it was the year 1000 on the Roman calendar!
We are probably celebrating the 2,000 the birthday of Jesus Christ 4 years too late.
If we were Roman we would not be celebrating the year 2000 AD but the year 2753
A calendar is the division of the year into months, weeks, and days and the method of ordering the years. From year one, an assumed date of the birth of Jesus, dates are calculated backwards (BC 'before Christ' or BCE 'before common era') and forwards (AD, Latin Anno Domini 'in the year of the lord' , or CE 'common era'). The lunar month (period between one new moon and the next) naturally averages 29.5 days, but the Western calendar uses for convenience a calendar month with a complete number of days, 30 or 31 (Feb has 28). For adjustments, since there are slightly fewer than six extra hours a year left over, they are added to Feb as a 29th day every fourth year (leap year), century years being excepted unless they are divisible by 400. For example, 1896 was a leap year. 1900 was not. 2000 is a leap year.
The month names in most European languages were probably derived as follows: January from Janus, Roman god; February from Februar, Roman festival of purification; March from Mars, Roman god; April from Latin aperire, 'to open'. May from Maia, Roman goddess; June from Juno, Roman goddess; July from Julius Caesar, Roman general; August from Augustus, Roman emperor; September, October, November, December (originally the seventh-tenth months) from the Latin words meaning seven, eight, nine and ten, respectively.
Days of the week
The days of the week are Monday named after the Moon; Tuesday from Tiu or Tyr, Anglo Saxon or Norse god; Wednesday from Woden or Odin, Norse god; Thursday from Thor, Norse god; Friday from Freya, Norse goddess; Saturday from Saturn, Roman god; and Sunday named after the Sun.
All early calendars except the ancient Egyptian were lunar. The word calendar comes from the Latin Kalendae or calendae, the first day of each month on which, in ancient Rome, solemn proclamation was made of the appearance of the new moon.
The Western or Gregorian calendar derives from the Julian calendar instituted by Julius Caesar 46 BC.
It was adjusted by Pope Gregory XIII in 1582, who eliminated the accumulated error caused by a faulty calculation of the length of a year and avoided its recurrence by restricting century leap years to those divisible by 400. Other states only gradually changed from Old Style to New Style; Britain and its colonies adopted the Gregorian calendar in 1752, when the error amounted to 11 days, and 3 Sept 1752 became 14 Sept (at the same time the beginning of the year was put back from 25 March to 1 Jan).
Russia did not adopt it until the October Revolution of 1917, so that the event (then 25 Oct) is currently celebrated 7 Nov.
Other Calendar Systems
The Jewish calendar is a complex combination of lunar and solar cycles, varied by considerations of religious observance. A year may have 12 or 13 months, each of which normally alternates between 29 and 30 days; the New Year (Rosh Hashanah) falls between 5 Sept and 5 Oct. The calendar dates from the hypothetical creation of the world (taken as 7 Oct 3761 BC). It was introduced until the 10th century (of the Western Calendar).
The Chinese calendar is lunar, with a cycle of 60 years. Both the traditional and, from 1911, the Western calendar are in use in China.
The Muslim calendar, also lunar, has 12 months of alternately 30 and 29 days, and a year of 354 days. This results in the calendar rotating around the seasons in a 30- year cycle.