One thing you can do is have more than one trigger, and leverage the Data Simulator tags.
For example, there are some square wave / sin wave tags in there for various periods. You could use a 30 minute sine wave and check for when it’s near zero. That would be true only every 15 minutes. You can “And” that condition w/the one you’re now checking to only have it potentially be true ever 15 minutes.
Do you see what I’m saying here?