I think you would have to filter the combo list with a query or sql that checks if the user entered date falls between any start and end date for that equipment. You could build and test this query and verify it works correctly before setting the combo row source to that query. There might be other options, such as a subform showing the in/out dates for the chosen equipment. Then users could see what's available at a glance.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.