I am not sure I am completely following - I think you mixed two ideas together.
If you want to not poll during specific times, then you would use "Unmanage Scheduling Utility" to stop polling (in start menu).
But if you want to poll them all the time, but you want to alert based on "on between 8:00am and 6:00pm" and "off between 6:00pm and 8:00am", you would create two alerts that query for that status with the time restriction.
Sorry, I probably mixed up terminology. I think I want the former... I don't want the nodes to be polled at all during a period of time, because they are intentionally offline and thus the polling would just generate false negatives. It sounds like the Unmanage Scheduling Utility will do what I want, thanks!