From the mIRC 6.16 help file: (/help /timer)
/timer[N/name] [-ceomhipr] [color:red][time] <repetitions> <interval> <command>[/color]
Activates the specified timer to perform the specified command at a specified interval, and optionally at a specified time.
/timer9 14:30 1 1 /say It's now 2:30pm
This will wait until 2:30pm and will then announce the time once and stop.
I guess it solves your problem.