Date

        # Date < Object

---
# Includes:
Comparable (from ruby core)

(from ruby core)
---
date and datetime class - Tadayoshi Funaba 1998-2011

'date' provides two classes: Date and DateTime.

## Terms and Definitions

Some terms and definitions are based on ISO 8601 and JIS X 0301.

### Calendar Date

The calendar date is a particular day of a calendar year, identified by
its ordinal number within a calendar month within that year.

In those classes, this is so-called "civil".

### Ordinal Date

The ordinal date is a particular day of a calendar year identified by
its ordinal number within the year.

In those classes, this is so-called "ordinal".

### Week Date

The week date is a date identified by calendar week and day numbers.

The calendar week is a seven day period within a calendar year, starting
on a Monday and identified by its ordinal number within the year; the
first calendar week of the year is the one that includes the first
Thursday of that year. In the Gregorian calendar, this is equivalent to
the week which includes January 4.

In those classes, this is so-called "commercial".

### Julian Day Number

The Julian day number is in elapsed days since noon (Greenwich Mean
Time) on January 1, 4713 BCE (in the Julian calendar).

In this document, the astronomical Julian day number is the same as the
original Julian day number. And the chronological Julian day number is a
variation of the Julian day number. Its days begin at midnight on local
time.

In this document, when the term "Julian day number" simply appears, it
just refers to "chronological Julian day number", not the original.

In those classes, those are so-called "ajd" and "jd".

### Modified Julian Day Number

The modified Julian day number is in elapsed days since midnight
(Coordinated Universal Time) on November 17, 1858 CE (in the Gregorian
calendar).

In this document, the astronomical modified Julian day number is the
same as the original modified Julian day number. And the chronological
modified Julian day number is a variation of the modified Julian day
number. Its days begin at midnight on local time.

In this document, when the term "modified Julian day number" simply
appears, it just refers to "chronological modified Julian day number",
not the original.

In those classes, those are so-called "amjd" and "mjd".

## Date

A subclass of Object that includes the Comparable module and easily
handles date.

A Date object is created with Date::new, Date::jd, Date::ordinal,
Date::commercial, Date::parse, Date::strptime, Date::today,
Time#to_date, etc.

    require 'date'

    Date.new(2001,2,3)
     #=> #<Date: 2001-02-03 ...>
    Date.jd(2451944)
     #=> #<Date: 2001-02-03 ...>
    Date.ordinal(2001,34)
     #=> #<Date: 2001-02-03 ...>
    Date.commercial(2001,5,6)
     #=> #<Date: 2001-02-03 ...>
    Date.parse('2001-02-03')
     #=> #<Date: 2001-02-03 ...>
    Date.strptime('03-02-2001', '%d-%m-%Y')
     #=> #<Date: 2001-02-03 ...>
    Time.new(2001,2,3).to_date
     #=> #<Date: 2001-02-03 ...>

All date objects are immutable; hence cannot modify themselves.

The concept of a date object can be represented as a tuple of the day
count, the offset and the day of calendar reform.

The day count denotes the absolute position of a temporal dimension. The
offset is relative adjustment, which determines decoded local time with
the day count. The day of calendar reform denotes the start day of the
new style. The old style of the West is the Julian calendar which was
adopted by Caesar. The new style is the Gregorian calendar, which is the
current civil calendar of many countries.

The day count is virtually the astronomical Julian day number. The
offset in this class is usually zero, and cannot be specified directly.

A Date object can be created with an optional argument, the day of
calendar reform as a Julian day number, which should be 2298874 to
2426355 or negative/positive infinity. The default value is
`Date::ITALY` (2299161=1582-10-15). See also sample/cal.rb.

    $ ruby sample/cal.rb -c it 10 1582
        October 1582
     S  M Tu  W Th  F  S
        1  2  3  4 15 16
    17 18 19 20 21 22 23
    24 25 26 27 28 29 30
    31

    $ ruby sample/cal.rb -c gb  9 1752
       September 1752
     S  M Tu  W Th  F  S
           1  2 14 15 16
    17 18 19 20 21 22 23
    24 25 26 27 28 29 30

A Date object has various methods. See each reference.

    d = Date.parse('3rd Feb 2001')
                                 #=> #<Date: 2001-02-03 ...>
    d.year                       #=> 2001
    d.mon                        #=> 2
    d.mday                       #=> 3
    d.wday                       #=> 6
    d += 1                       #=> #<Date: 2001-02-04 ...>
    d.strftime('%a %d %b %Y')    #=> "Sun 04 Feb 2001"


---
# Constants:

ABBR_DAYNAMES
:   An array of strings of abbreviated day names in English.  The first
    is "Sun".
ABBR_MONTHNAMES
:   An array of strings of abbreviated month names in English.  The
    first element is nil.
DAYNAMES
:   An array of strings of the full names of days of the week in
    English. The first is "Sunday".
ENGLAND
:   The Julian day number of the day of calendar reform for England and
    her colonies.
GREGORIAN
:   The Julian day number of the day of calendar reform for the
    proleptic Gregorian calendar.
ITALY
:   The Julian day number of the day of calendar reform for Italy and
    some catholic countries.
JULIAN
:   The Julian day number of the day of calendar reform for the
    proleptic Julian calendar.
MONTHNAMES
:   An array of strings of full month names in English.  The first
    element is nil.


# Class methods:

    _httpdate
    _iso8601
    _jisx0301
    _parse
    _rfc2822
    _rfc3339
    _rfc822
    _strptime
    _xmlschema
    civil
    commercial
    gregorian_leap?
    httpdate
    iso8601
    jd
    jisx0301
    json_create
    julian_leap?
    leap?
    new
    new!
    nth_kday
    ordinal
    parse
    rfc2822
    rfc3339
    rfc822
    strptime
    test_all
    test_civil
    test_commercial
    test_nth_kday
    test_ordinal
    test_unit_conv
    test_weeknum
    today
    valid_civil?
    valid_commercial?
    valid_date?
    valid_jd?
    valid_ordinal?
    weeknum
    xmlschema

# Instance methods:

    +
    -
    <<
    <=>
    ===
    >>
    ajd
    amjd
    as_json
    asctime
    ctime
    cwday
    cweek
    cwyear
    day
    day_fraction
    downto
    england
    fill
    friday?
    gregorian
    gregorian?
    hour
    httpdate
    infinite?
    inspect
    inspect_raw
    iso8601
    italy
    jd
    jisx0301
    julian
    julian?
    ld
    leap?
    marshal_dump_old
    mday
    min
    minute
    mjd
    mon
    monday?
    month
    new_start
    next
    next_day
    next_month
    next_year
    nth_kday?
    prev_day
    prev_month
    prev_year
    rfc2822
    rfc3339
    rfc822
    saturday?
    sec
    second
    start
    step
    strftime
    succ
    sunday?
    thursday?
    to_date
    to_datetime
    to_json
    to_s
    to_time
    tuesday?
    upto
    wday
    wednesday?
    wnum0
    wnum1
    xmlschema
    yday
    year