Date/Time extensions calculating various permutations of dates.

date_extensions.rb

require 'date'

module DateExtensions
  
  WEEKDAYS = { :sunday => 0, :monday => 1, :tuesday => 2, :wednesday => 3,
               :thursday => 4, :friday => 5, :saturday => 6 }
  
  # Returns the first weekday of the month.
  def first_wday
    (wday - day + 1) % 7
  end
  
  # Returns the day of the month for the +(n\+1)+-th weekday +d+ of this month.  The first week is +0+.
  def day_of_nwday(n,d)
    1 + (d - (wday - day + 1)) % 7 + n * 7
  end

  # Returns the day of the month for the +(n)+-th weekday +d+ of this month.
  # If not supplied, +d+ defaults to wday. The first week is +1+.
  def day_of_nth_wday(n,d=wday)
    day_of_nwday n-1, WEEKDAYS[d]||d
  end
  
  # Returns an instance representing the soonest +(n)+-th weekday +d+ of month +m+,
  # such that +new_instance >= self+ _and_ +new_instance < self.next_year+.
  # If not supplied, +d+ defaults to wday, and +m+ defaults to month.
  def soonest_nth_wday_of_month(n,d=wday,m=month)
    m = month + 12 if m == month && day_of_nth_wday(n,d) < day
    months_since(m - month).at_nth_wday(n, d)
  end
  alias soonest_nth_wday soonest_nth_wday_of_month

  # Week of the year, starting at zero.
  def yweek
    (yday - wday) / 7 + 1
  end

  # Week of the month, starting at zero.
  def mweek
    (5 - wday + day) / 7
  end

  # Return the day name.
  def day_name
    Date::DAYNAMES[wday]
  end

  # Return the abbreviated day name.
  def abbr_day_name
    Date::ABBR_DAYNAMES[wday]
  end

  # Return the month name.
  def month_name
    Date::MONTHNAMES[month]
  end

  # Return the abbreviated month name.
  def abbr_month_name
    Date::ABBR_MONTHNAMES[month]
  end
end

class Date
  include DateExtensions
  
  # Returns a new instance for the day of this month returned by +day_of_nwday+.
  def at_nwday(n,d=wday)
    self + day_of_nwday(n, d) - day
  end
  
  # Returns a new instance for the day of this month returned by +day_of_nth_wday+.
  def at_nth_wday(n,d=wday)
    self + day_of_nth_wday(n, d) - day
  end
end

class Time
  include DateExtensions
  
  # Returns a new instance for the day of this month returned by +day_of_nwday+.
  def at_nwday(n,d=wday)
    self + day_of_nwday(n, d).days - day.days
  end
  
  # Returns a new instance for the day of this month returned by +day_of_nth_wday+.
  def at_nth_wday(n,d=wday)
    self + day_of_nth_wday(n, d).days - day.days
  end
end