I also had this problem with my 2011 Forte EX after the not so reliable digital clock had the problems notorious for Fortes. I found the fix for the clock on kiatechinfo.com, it was the Technical Service Bulletin called "Service Action: Forte Digital Clock Adjustment (SA073)". However, this did nothing for me for the temperature as the fix for the clock changes the temperature display to F. The manual was useless for anything but the SX as the LX and EX have manual temperature controls. This forum informed me how to change the temperature from F to C. Thanks.
By the way, I took my car to the Kia dealer near where I live and they apparently "fixed" the problem, but this is now the second time that the clock has messed up. I will monitor, but if this does it again, obviously the TSB from Kia is not a permanent solution and Kia will need to come up with a better and permanent fix. This should not be happening for a brand new car that I have only had for one month and only has about 4000 kms on it.