If you go to this web site for Toro brand timers, you will get info on a timer that allows you to use your computer to set each of your zones - based on the weather (temperature/humidity). I believe you can just download the software and then manually adjust your zones instead of buying the new timer. The program asks questions about your average high temperature, the type of sprinkler heads you have in each zone, etc. It then prints out a weekly schedule for each zone.
I suppose it assumes that your system is properly installed and each zone is operating at its optimum level. It only makes sense to water according to the weather. Why would you want to water the same when the average temp. is 50 degrees as when the average temp. is 90-95 degrees?
Toro EC XTRA Timer