The plugin is quite simple - it displays a progressbar showing the percentage
of the time elapsed. Left-clicking on the plugin area opens a menu of available
alarms. After selecting one, the user can start or stop the timer by selecting
start/stop timer entry in the same menu. New alarms are added through the
preferences window. Each alarm is either a countdown or is run at a specified
time. By default a simple dialog pops up at the end of the countdown. The user
can choose an external command to be run as the alarm and may also choose to
have this repeated a specified number of times with a given interval between
repetitions.

WWW: https://goodies.xfce.org/projects/panel-plugins/xfce4-timer-plugin