That will not work for recurring downtimes. Simonz wants to schedule a down time that starts at say midnight and lasts for 10 minutes but to have that downtime stay current for everyday. With what Hiii00 pointed too will not work for this situation. Nagios' downtime function assumes that the downtime is a one-time thing or something that is planned many days in advance and will not reoccur for an unknown time.
The only way I know to do that is like I said above. Exclude your downtime from your timeperiod. Here is an example that works for 2.x
alias Server that needs to be rebooted - must be windows
This will effectively make from midnight to 12:15am a no notify time.