Programming example

add_minutes.rb

class IncorrectTimeFormatError < StandardError; end

class FakeTime
  ##
  #  :call-seq:
  #     FakeTime.add_minutes(time, minutes_to_add)  =>  String
  #  
  #  Returns a new string with minutes_to_add added to the time
  #     
  #     FakeTime.add_minutes("9:23 AM", 10)   #=> "9:33 AM"
  #     
  #
  def self.add_minutes(time, minutes_to_add)
    if matches = time.match(/\A(1[0-2]|\d):(\d{2}) (AM|PM)\z/i)

      #split the time string
      all, hours, minutes, ampm = matches.to_a

      #sanitize hours/ampm
      ampm.upcase!
      hours = hours.to_i

      #convert to 24 hour format
      if ampm.match(/PM/)
        hours += 12
      end

      #add minutes to running total
      total_minutes = minutes.to_i + minutes_to_add.to_i
      minutes = total_minutes % 60

      #calculate new total for hours
      hours = hours + total_minutes / 60

      #convert back to 12 hour format
      if hours % 24 / 12 == 1
        ampm = "PM"
      else
        ampm = "AM"
      end

      hours = hours % 12
      hours = hours == 0 ? 12 : hours

      "#{hours}:#{"%02d" % minutes} #{ampm}"
    else
      raise IncorrectTimeFormatError.new("#{time} is not in the format [H]H:MM [AM|PM]")
    end
  end
end