def add_exception_time(time)
      return if time.nil?
      rule = SingleOccurrenceRule.new(time)
      add_exception_rule rule
      time
    end
def add_exception_time(time)
      return if time.nil?
      rule = SingleOccurrenceRule.new(time)
      add_exception_rule rule
      time
    end
Add an exception time to the schedule