If you want to be reminded of something that happens only once (meeting, date, see dentist, lawyer...) then you need to make a Onetime event. After that date (and time if alarm is set), that event expires. To make a Onetime event you need to enter all fields (dd, mm and yyyy). In this example, on March 15th year 2000 at 11:15h you will get sound alarm that repeats every 5 minutes (other alarm actions can be set also). On that day, and the day before (Days Notice = 1) you will get warning of upcoming event in Viewer.

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