== 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 countries 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

