How can you build a short range 72 MHz wireless clock transmitter?

My school uses Primex Wireless clocks across the entire building. There's a single receiver on one side of the school, and I'm quite curious about how it works. I found some white papers on the transmitters, which gave me frequencies. (http://www.primexwireless.com/our_products/gps_wireless_system/clocks_and_more/xr_transmitters/)

What I'm trying to do is create a small transmitter that does the same thing in a very short range. I don't know how to figure out what sort of data is considered a valid time code. I've seen the instructables on making computer controlled radio transmitters, and I'd like to figure out how to adapt these to this situation. Different channels for example, how do these receivers pick up different channels within the 72 MHz range?