The Gregorian calendar is the calendar used in most countries around the world. It was proposed by Aloysius Lilius, and was adopted by Pope Gregory XIII, February 24, 1582 as a minor reform of the Julian calendar. Sweden introduced Gregorian calendar year 1753.
Average year in the Julian calendar was 365.25 days long, but since the average year is 365.2422 days, came the calendar eventually out of step. In the Julian calendar was a leap year if they had the year evenly divisible by four. In the Gregorian calendar is not even centuries leap years unless they are not evenly divisible by 400. This means that, for example, the years 1800 and 1900 were not leap years, but 2000 was a leap year. The Gregorian calendar receives an average of 365.2425 days in a year, which gives an error of only one day per 3236 years.
The years in the Gregorian calendar counts for presumptive Christ's birth, which the calendar era, the first year, the year 1. Year 0 is the year in which the traditional year form does not exist. Year 1 AD according to the Gregorian calendar follows directly after year 1 f.Kr. The abbreviations "AD" or "BC" used in parallel with the vt or BCE to clarify that the year relates to the respective prior epoch; without shortening it is normally understood that the years after the epoch referred. The birth of Christ is believed to have been estimated wrong with a few years of Jesus according to recent calculations must be born in 7-4 BC, but the system is incorporated so that it is still used. The incorrect calculation of the birth of Christ was made by the abbot Dionysius Exiguus who in 525 was commissioned by the Pope to further pinpoint the time of the Christian Easter, this time without the year that relate to a specific regents to the throne, as there has been since time immemorial. An earlier attempt to determine time for Easter was made at the Council of Nicaea in 325, then the principle of "first Sunday after the first full moon after the vernal equinox."
First century BC includes 1 BC to 100 BC, the second century 101 BC to 200 BC and so on. Are thus added back in time by the epoch in this way. Note that we now, for practical reasons, usually count the 100 years starting on the same figures for a century, which, if it interpolates backward, would lead to the first century BC and the first AD, contains only 99 years.
How to denote centuries vary in different countries. In the UK it is estimated the number of centuries. Eg the years 1-100 "The First Century". Therefore, what we call the 1800s, for example, the same as the "19th century" in the UK.
In other calendars counted the years on the basis of other events. The Muslim calendar begins July 16 622 AD (Muhammad's flight to Medina), the Roman calendar began on April 21 753 BC (Founding of Rome) and the Hebrew calendar begins October 7 3761 BC (Creation of Adam).
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