When AHP sets the CM15A clock it always sets it to the whole minute, disregarding the seconds. E.g., if the actual time is
8:30:45 the clock will be set to 8:30:00. So your downloaded timers can execute as much as 59 seconds late.
After downloading your timers, if you wait until your PC clock just rolls over to the next minute and then click Tools > Hardware configuration, then click Cancel, you can set the CM15A clock to within a second or two of the actual PC time.
I can't account for your timer running 5 minutes late.