
               [----------]
        <-->                            - -
        <-------------->                - 0   y
        <----------------------->       - +   y
                  <---->                0 0   y
                  <------------->       0 +   y
                             <-->       + +


Datebook contains DatebookEvents
  events
  listeners
  addEvent()
  save()

DatebookEvent
  beginTime
  endTime
  alarm
  dispose()
  addAlarm()
  setTimes()

alarm
  time
  action

action
  + dialog
  + command
