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62 lines
2.1 KiB
Text
62 lines
2.1 KiB
Text
== Julian and Gregorian Calendars
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The difference between the
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{Julian calendar}[https://en.wikipedia.org/wiki/Julian_calendar]
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and the
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{Gregorian calendar}[https://en.wikipedia.org/wiki/Gregorian_calendar]
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may matter to your program if it uses dates before the switchovers.
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- October 15, 1582.
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- September 14, 1752.
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A date will be different in the two calendars, in general.
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=== Different switchover dates
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The reasons for the difference are religious/political histories.
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- On October 15, 1582, several countries changed
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from the Julian calendar to the Gregorian calendar;
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these included Italy, Poland, Portugal, and Spain.
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Other contries in the Western world retained the Julian calendar.
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- On September 14, 1752, most of the British empire
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changed from the Julian calendar to the Gregorian calendar.
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When your code uses a date before these switchover dates,
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it will matter whether it considers the switchover date
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to be the earlier date or the later date (or neither).
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See also {a concrete example here}[rdoc-ref:DateTime@When+should+you+use+DateTime+and+when+should+you+use+Time-3F].
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=== Argument +start+
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Certain methods in class \Date handle differences in the
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{Julian and Gregorian calendars}[rdoc-ref:calendars.rdoc@Julian+and+Gregorian+Calendars]
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by accepting an optional argument +start+, whose value may be:
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- Date::ITALY (the default): the created date is Julian
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if before October 15, 1582, Gregorian otherwise:
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d = Date.new(1582, 10, 15)
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d.prev_day.julian? # => true
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d.julian? # => false
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d.gregorian? # => true
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- Date::ENGLAND: the created date is Julian if before September 14, 1752,
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Gregorian otherwise:
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d = Date.new(1752, 9, 14, Date::ENGLAND)
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d.prev_day.julian? # => true
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d.julian? # => false
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d.gregorian? # => true
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- Date::JULIAN: the created date is Julian regardless of its value:
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d = Date.new(1582, 10, 15, Date::JULIAN)
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d.julian? # => true
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- Date::GREGORIAN: the created date is Gregorian regardless of its value:
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d = Date.new(1752, 9, 14, Date::GREGORIAN)
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d.prev_day.gregorian? # => true
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