So using this data
John SMith 7:00 15:00
Jane Smith 7:30 15:30
Tom Smith 8:00 16:00
Tara Smith 10:00 18:00
the agent count between 8:00 and 8:30 AM is 4? At 16:00 it is 2?
You would not check from 8:30 to 9:00 as well as 9:00 to 9:30 but rather 8:30 to 8:59 and 9:00 to 9:29 or something similar?
A form with a timer event could run a query (or set of) every 30 minutes. Rather than rely on the system clock staying in sync with some starting point so that your interval check is what you want, I think I'd have a table of frequencies of start and ends (2 additional fields) and a sequence field (1 to n, rather than an autonumber ID field). A form level variable could start with 1 (sequence 1) and every time you run the test, it increments by 1 so that the next time you use the next sequence (2, which is 8:30 to 8:59). You should be able to get a count of all the scheduled people WHERE Start >= 8:30 AND End <= 8:59 (where those time values come from the record for sequence 2) by using a Totals query.
Not sure if that is as clear as mud or what.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.