i have 3 advanced alerts that are setup like this on the same devices:
12am - 7am = send an alert if device is not equal to up after 1 hour
7am - 6pm = (duty hours) send an alert immediately if device is not equal to up
6pm - 12pm = send an alert if device is not equal to up after 1 hour
low and behold a device went down at 11:58 last night triggering an alert. then i got another at 1am indicating that it had in fact been down for an hour. then i got another at 7am saying it had been down an hour again