The rain bird ESP series clocks are what I am most familiar with. They have three programs with four avalible start time settings. If you set the zone time in all three programs then set all four start times for each program that will give you 12 starts.

You could also use a propagation timer.

ann,

Your description is a little confusing. What do you mean by cycle of 15 minutes every hour. Each irrigation rotor head runs for a length of time and you need to turn on every rotor head in a cycle every hour with the 15 minutes divided amongst the heads? How many heads do you need to operate? Is your climate very arid? Aloha

Thanks everyone for the responses. We were able to get the Hunter Pro working again and in doing so discovered it has the 3 programs that mike mentioned is on the Rainbirds. This unit belongs to my DS so I was not familiar with it until I got hold of the manual. Still took mike's description for me to grasp the manual's instructions!
There are 2 zones on the controller and the start time for one of the programs is determined by the other zone/valve. Because of this I was limited to 9 starts vs a max of 12 for the one zone.

twolips......I've never heard of the propagation timer but will keep it in mind for future projects. Thanks!

lehua......we have a total of 6 rotators on one zone. I wanted all of them to come on at the same time and run for a set time. They're now coming on every 1 1/2 hrs and running for 5 min. Sorry for the confusion.
We're in the PNW so the climate is not arid but there are a number of windy days which make for dusty conditions in the paddocks and the barn. So far, the system is working perfectly and I can't say enough good things about Hunter MP Rotators.