I still think this is the wrong approach from a user friendly aspect because the OP suggests this scenario is likely:
It seems that the dates are to be entered first, then choose the equipment, so I enter the 2 dates for which I need some particular equipment.
Drop down the combo, my equipment isn't listed. OK, it's booked out? Or do I have absolute confidence that there's nothing wrong with the combo query?
Now I move the dates forward 3 days. No luck.
Move the dates forward 4 more days. No luck.
Move the dates backward 4 days from the original. No luck
Move the dates 2 weeks forward from original. No luck
Move the dates ...
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.