proleptic Gregorian calendar

The proleptic Gregorian calendar is produced by extending the Gregorian calendar to dates preceding its official introduction in 1582.Most Maya scholars use the proleptic Gregorian calendar, especially when converting Long Count dates (first century BC to tenth century). It is also explicitly required for all dates before 1582 by ISO8601:2004 (4.3.2.1), if the partners to information exchange agree. However, neither astronomers nor non-Maya historians generally use it.
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