various_date_functions.rb

    # Compute previous scheduled date.

    def get_previous_day(date, day_of_week)
      date = date.to_date
      date - ((date.wday - day_of_week) % 7)
    end

    def get_previous_month_day(date, day_of_month)
      date = date.to_date

      if date.mday <= day_of_month
        date.beginning_of_month + (day_of_month - 1).days - 1.month
      else
        date.beginning_of_month + (day_of_month - 1).days
      end
    end

    def get_previous_2week_day(date, start_date, day_of_week)
      date                  = date.to_date
      previous_day          = date - ((date.wday - day_of_week) % 7)
      previous_previous_day = previous_day - 7.days

      start_date            = start_date.to_date
      weeks                 = ((date - start_date.to_date)/7).to_i
      week_multiplier       = (weeks % 2) == 0

      # puts "#{date} #{start_date} #{week_multiplier} #{previous_day} #{previous_previous_day}"

      if date >= start_date
        if week_multiplier
          date.wday == day_of_week ? date : previous_day
        else
          date.wday == day_of_week ? date - 7.days : previous_previous_day
        end
      else
        # ???
      end
    end
    # get_previous_2week_day(Date.new(2016,2,19),Date.new(2016,2,18),4)

    # Compute next scheduled date.

    def get_next_day(date, day_of_week)
      date = date.to_date
      date + ((day_of_week - date.wday) % 7)
    end

    def get_next_month_day(date, day_of_month)
      date = date.to_date

      if date.mday <= day_of_month
        date.beginning_of_month + (day_of_month - 1).days
      else
        date.beginning_of_month + (day_of_month - 1).days + 1.month
      end
    end

    # Compute next day in two weeks interval from start_date.
    # date - current day (today)
    # start_date - date from which interval started
    # day_of_week - day of week as number monday = 1 ... sunday = 0
    def get_next_2week_day(date, start_date, day_of_week)
      date              = date.to_date
      next_day          = date + ((day_of_week - date.wday) % 7)
      next_next_day     = next_day + 7.days

      start_date        = start_date.to_date
      weeks             = ((date - start_date.to_date)/7).to_i
      week_multiplier   = (weeks % 2) == 0

      # puts "#{date} #{start_date} #{week_multiplier} #{next_day} #{next_next_day}"

      if date >= start_date
        if week_multiplier
          date.wday == day_of_week ? date : next_next_day
        else
          date.wday == day_of_week ? next_next_day : next_day
        end
      else
        # ???
      end
    end
    # get_next_2week_day(Date.new(2016,2,5),Date.new(2016,2,4),4)