The reminder time for all day events is not configurable.

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Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours
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>i can set the reminder for timed events, but tend to use all day events as
> reminders. at the moment the default reminder is at 18 hours, which is
> not
> useful to me, so i would like to change this please.