I expect this post to be deleted if everyone already knows this ... ...but I just discovered it today.

In addition to dueBefore:Today, you can also search dueBefore:Now -- which means I can have my lunchtime To-Do's appear on a SmartList only after lunch lunchtime (if I enter a starttime for those items, of course.)

This is especially great for me who hates to see my to do list cluttered with things that I can't do or shouldn't be doing at the present time.