Sounds a bit like this question from a recovering ladder logic user.
I think both could use the Start Off-Pulse or Start On-Pulse commands, which get sent to the brain then move along in the next step of your chart.
At that point, the brain (I/O Unit) is doing the timing, so if you send no other command to that point, it will turn on (in the case of the Start Off-Pulse command) when the time you specified is up.
However, if you do send another Start/Stop-Pulse command, the brain will be start counting over w/the new Off/On time. Or if you just Turn On/Off the point, your pulse command will get cancelled.
Make sense? Will that work for you?