I see this on rare occasions but have not been able to repeat the problem enough to diagnose it.  Something causes the processor to lock up.  The good news is that it rarely occurs and is easily fixed by doing the following:  (You might want to write down your current schedule before doing this since the Irritrol RD600 will default program to 7am, every station, 10minutes on schedule A.)

Perform a complete power-down reset by:

1. remove the battery
2. pull off the ribbon cable from the back connector board.
3. Wait at lease a minute for any residual power to completely drain from the unit
4. Plug in the ribbon cable again (it should power up)
5. reconnect the battery.

It should start with a blinking 12:00pm and now work as expected; you can now set the time.

If this doesn’t work, try following the more detailed instructions on this page.  Note that it has additional factory reset settings for the Irritrol RD-xxx-R series of controllers.

Good luck,

Brian