Sounds a bit like [B][U]this question[/U][/B] from a recovering ladder logic user.
I think both could use the [B]Start Off-Pulse[/B] or [B]Start On-Pulse[/B] commands, which get sent to the [I]brain [/I]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 [B]Start Off-Pulse [/B]command) when the time you specified is up.
However, if you do send another [B]Start/Stop-Pulse[/B] command, the brain will be start counting over w/the new Off/On time. Or if you just [B]Turn On/Off[/B] the point, your pulse command will get cancelled.
Make sense? Will that work for you?