calculations.rb

require 'date'
require 'active_support/duration'
require 'active_support/core_ext/object/acts_like'
require 'active_support/core_ext/date/zones'
require 'active_support/core_ext/time/zones'

class Date
  if RUBY_VERSION < '1.9'
    undef :>>
    
    # Backported from 1.9. The one in 1.8 leads to incorrect next_month and
    # friends for dates where the calendar reform is involved. It additionally
    # prevents an infinite loop fixed in r27013.
    def >>(n)
      y, m = (year * 12 + (mon - 1) + n).divmod(12)
      m,   = (m + 1)                    .divmod(1)
      d = mday
      until jd2 = self.class.valid_civil?(y, m, d, start)
        d -= 1
        raise ArgumentError, 'invalid date' unless d > 0
      end
      self + (jd2 - jd)
    end
  end

  class << self
    # Returns a new Date representing the date 1 day ago (i.e. yesterday's date).
    def yesterday
      ::Date.current.yesterday
    end

    # Returns a new Date representing the date 1 day after today (i.e. tomorrow's date).
    def tomorrow
      ::Date.current.tomorrow
    end

    # Returns Time.zone.today when config.time_zone is set, otherwise just returns Date.today.
    def current
      ::Time.zone_default ? ::Time.zone.today : ::Date.today
    end
  end

  # Tells whether the Date object's date lies in the past
  def past?
    self < ::Date.current
  end

  # Tells whether the Date object's date is today
  def today?
    self.to_date == ::Date.current # we need the to_date because of DateTime
  end

  # Tells whether the Date object's date lies in the future
  def future?
    self > ::Date.current
  end

  # Converts Date to a Time (or DateTime if necessary) with the time portion set to the beginning of the day (0:00)
  # and then subtracts the specified number of seconds
  def ago(seconds)
    to_time_in_current_zone.since(-seconds)
  end

  # Converts Date to a Time (or DateTime if necessary) with the time portion set to the beginning of the day (0:00)
  # and then adds the specified number of seconds
  def since(seconds)
    to_time_in_current_zone.since(seconds)
  end
  alias :in :since

  # Converts Date to a Time (or DateTime if necessary) with the time portion set to the beginning of the day (0:00)
  def beginning_of_day
    to_time_in_current_zone
  end
  alias :midnight :beginning_of_day
  alias :at_midnight :beginning_of_day
  alias :at_beginning_of_day :beginning_of_day

  # Converts Date to a Time (or DateTime if necessary) with the time portion set to the end of the day (23:59:59)
  def end_of_day
    to_time_in_current_zone.end_of_day
  end

  def plus_with_duration(other) #:nodoc:
    if ActiveSupport::Duration === other
      other.since(self)
    else
      plus_without_duration(other)
    end
  end
  alias_method :plus_without_duration, :+
  alias_method :+, :plus_with_duration

  def minus_with_duration(other) #:nodoc:
    if ActiveSupport::Duration === other
      plus_with_duration(-other)
    else
      minus_without_duration(other)
    end
  end
  alias_method :minus_without_duration, :-
  alias_method :-, :minus_with_duration

  # Provides precise Date calculations for years, months, and days.  The +options+ parameter takes a hash with
  # any of these keys: <tt>:years</tt>, <tt>:months</tt>, <tt>:weeks</tt>, <tt>:days</tt>.
  def advance(options)
    options = options.dup
    d = self
    d = d >> options.delete(:years) * 12 if options[:years]
    d = d >> options.delete(:months)     if options[:months]
    d = d +  options.delete(:weeks) * 7  if options[:weeks]
    d = d +  options.delete(:days)       if options[:days]
    d
  end

  # Returns a new Date where one or more of the elements have been changed according to the +options+ parameter.
  #
  # Examples:
  #
  #   Date.new(2007, 5, 12).change(:day => 1)                  # => Date.new(2007, 5, 1)
  #   Date.new(2007, 5, 12).change(:year => 2005, :month => 1) # => Date.new(2005, 1, 12)
  def change(options)
    ::Date.new(
      options[:year]  || self.year,
      options[:month] || self.month,
      options[:day]   || self.day
    )
  end

  # Returns a new Date/DateTime representing the time a number of specified months ago
  def months_ago(months)
    advance(:months => -months)
  end

  # Returns a new Date/DateTime representing the time a number of specified months in the future
  def months_since(months)
    advance(:months => months)
  end

  # Returns a new Date/DateTime representing the time a number of specified years ago
  def years_ago(years)
    advance(:years => -years)
  end

  # Returns a new Date/DateTime representing the time a number of specified years in the future
  def years_since(years)
    advance(:years => years)
  end

  # Shorthand for years_ago(1)
  def prev_year
    years_ago(1)
  end unless method_defined?(:prev_year)

  # Short-hand for years_since(1)
  def next_year
    years_since(1)
  end unless method_defined?(:next_year)
  
  # Short-hand for months_ago(1)
  def prev_month
    months_ago(1)
  end unless method_defined?(:prev_month)

  # Short-hand for months_since(1)
  def next_month
    months_since(1)
  end unless method_defined?(:next_month)

  # Returns a new Date/DateTime representing the "start" of this week (i.e, Monday; DateTime objects will have time set to 0:00)
  def beginning_of_week
    days_to_monday = self.wday!=0 ? self.wday-1 : 6
    result = self - days_to_monday
    self.acts_like?(:time) ? result.midnight : result
  end
  alias :monday :beginning_of_week
  alias :at_beginning_of_week :beginning_of_week

  # Returns a new Date/DateTime representing the end of this week (Sunday, DateTime objects will have time set to 23:59:59)
  def end_of_week
    days_to_sunday = self.wday!=0 ? 7-self.wday : 0
    result = self + days_to_sunday.days
    self.acts_like?(:time) ? result.end_of_day : result
  end
  alias :sunday :end_of_week
  alias :at_end_of_week :end_of_week

  # Returns a new Date/DateTime representing the start of the given day in next week (default is Monday).
  def next_week(day = :monday)
    days_into_week = { :monday => 0, :tuesday => 1, :wednesday => 2, :thursday => 3, :friday => 4, :saturday => 5, :sunday => 6}
    result = (self + 7).beginning_of_week + days_into_week[day]
    self.acts_like?(:time) ? result.change(:hour => 0) : result
  end

  # Returns a new ; DateTime objects will have time set to 0:00DateTime representing the start of the month (1st of the month; DateTime objects will have time set to 0:00)
  def beginning_of_month
    self.acts_like?(:time) ? change(:day => 1,:hour => 0, :min => 0, :sec => 0) : change(:day => 1)
  end
  alias :at_beginning_of_month :beginning_of_month

  # Returns a new Date/DateTime representing the end of the month (last day of the month; DateTime objects will have time set to 0:00)
  def end_of_month
    last_day = ::Time.days_in_month( self.month, self.year )
    self.acts_like?(:time) ? change(:day => last_day, :hour => 23, :min => 59, :sec => 59) : change(:day => last_day)
  end
  alias :at_end_of_month :end_of_month

  # Returns a new Date/DateTime representing the start of the quarter (1st of january, april, july, october; DateTime objects will have time set to 0:00)
  def beginning_of_quarter
    beginning_of_month.change(:month => [10, 7, 4, 1].detect { |m| m <= self.month })
  end
  alias :at_beginning_of_quarter :beginning_of_quarter

  # Returns a new Date/DateTime representing the end of the quarter (last day of march, june, september, december; DateTime objects will have time set to 23:59:59)
  def end_of_quarter
    beginning_of_month.change(:month => [3, 6, 9, 12].detect { |m| m >= self.month }).end_of_month
  end
  alias :at_end_of_quarter :end_of_quarter

  # Returns a new Date/DateTime representing the start of the year (1st of january; DateTime objects will have time set to 0:00)
  def beginning_of_year
    self.acts_like?(:time) ? change(:month => 1, :day => 1, :hour => 0, :min => 0, :sec => 0) : change(:month => 1, :day => 1)
  end
  alias :at_beginning_of_year :beginning_of_year

  # Returns a new Time representing the end of the year (31st of december; DateTime objects will have time set to 23:59:59)
  def end_of_year
    self.acts_like?(:time) ? change(:month => 12,:day => 31,:hour => 23, :min => 59, :sec => 59) : change(:month => 12, :day => 31)
  end
  alias :at_end_of_year :end_of_year

  # Convenience method which returns a new Date/DateTime representing the time 1 day ago
  def yesterday
    self - 1
  end

  # Convenience method which returns a new Date/DateTime representing the time 1 day since the instance time
  def tomorrow
    self + 1
  end
end