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