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The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582. In countries that adopted the Gregorian calendar later, dates occurring in the interim (between 1582 and the local adoption) are sometimes "Gregorian-ized" as well.〔For example, George Washington was born on February 11, 1732 (Old Style), as Britain was using the Julian calendar. After the switch, that day became February 22, which is the date commonly given as Washington's birthday.〕 ==Usage== The proleptic Gregorian calendar is explicitly required for all dates before 1582 by ISO 8601:2004 (clause 4.3.2.1 The Gregorian calendar) if the partners to information exchange agree. It is also used by most Maya scholars, especially when converting Long Count dates (1st century BC – 10th century). However, neither astronomers nor non-Maya historians generally use it. The best practice for historians quoting contemporary documents is to give the date as in the original and to add as a note any contextual clues, conclusions about the calendar used and equivalents in other calendrical systems. This gives others the opportunity to re-evaluate the evidence.〔 Spathaky, Mike (Old Style New Style dates and the change to the Gregorian calendar ). "increasingly parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the New Style 1734 started even though the Old Style 1733 continued until 24th March. .. We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Old or the New Style reckoning. The date should either be written 20th January 1745 O.S. (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."〕 For these calendars one can distinguish two systems of numbering years BC. Bede and later historians did not use the Latin zero, ''nulla'', as a year (see 0 (year)), so the year preceding AD 1 is 1 BC. In this system the year 1 BC is a leap year (likewise in the proleptic Julian calendar). Mathematically, it is more convenient to include a year 0 and represent earlier years as negative, for the specific purpose of facilitating the calculation of the number of years between a negative (BC) year and a positive (AD) year. This is the convention used in astronomical year numbering and in the international standard date system, ISO 8601. In these systems, the year 0 is a leap year.〔 〕 Although the nominal Julian calendar began in 45 BC, leap years between 45 BC and 1 BC were irregular (see Leap year error). Thus the Julian calendar with quadrennial leap years was only used from the end of AD 4 until 1582 or later. The proleptic Gregorian calendar is sometimes used in computer software to simplify the handling of older dates. For example, it is the calendar used by MySQL,〔 (【引用サイトリンク】title= 11.8. What Calendar Is Used By MySQL? )〕 SQLite,〔 (【引用サイトリンク】title= Date And Time Functions )〕 PHP, CIM, Delphi, Python〔(【引用サイトリンク】title=8.1.3. date Objects )〕 and COBOL. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Proleptic Gregorian calendar」の詳細全文を読む スポンサード リンク
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