15 minutes delay - User Guide

scheduleNextTime is the dateTime of the next event

scheduleNextVal is the value of what the schedule will do for the next event. 

boolTest was only made so you can have a bool value that will be true during the period you want. In this case it will only be true when the current site time is greater than the next time minus 15 minutes. 
This will allow you to figure out how to do something 15 minutes before the schedule. Notice, I added an andGate too, cause you also want to make sure the next event is turning on, and not turning off.