Finding distinct values across multiple columns - FREQUENCY array?

Hi guys

I'm having trouble getting my head around array formulae, but I'm pretty sure that's what I need here.

I basically have a sheet with data in the following format:

Dept Agent Date
AM - hjhjlk Michael Jones 04/03/2014
AM - uyuiyi John Smith 05/03/2014
AM - hjhjlk Michael Jones 05/03/2014
AM - uyuiyi Mark Johnson 06/03/2014
JS - jhkkjhi Pierre Ginola 06/03/2014
AM - hjhjlk Michael Jones 06/03/2014
AM - hjhjlk Anne Frank 06/03/2014
JS - ghgjh Julio Ramirez 06/03/2014

What I need to do is count the number of distinct agents for a department (based only on the first two letters of the department) for a particular time period.
So, for instance:

Date range 04/03/14-06/03/14
AM would return 4
JS would return 2

Date range 04/03/14-05/03/14
AM would return 2
JS would return 0

I'm almost certain it's going to require an array formula using the FREQUENCY function, but despite trying a number of things I'm struggling to adapt other formulas I've found online to meet my requirements.

Any help would be massively appreciated.

Thanks XOR, works just as required!

Unfortunately it's looking like it'll be too slow given the amount of data I have (10k+ rows), so I'll probably need to go down the VBA route, but hopefully this helps someone else!