Same problem tried all above, does not work.
I noticed when it is connected with the computer or a power supply the time runs ok.
As soon you disconnect and go to standby you loose your time and it is back to the time you started with the computer or when you plugged in the power.
I have 6 collegues with an iPhone 5c with IOS 8.3 and they have no problems?
So I think it is a hardware problem and then it is the usual with Apple deny and no warrenty.
My iPhone has this problem for over a year and the answer is always wait for the update and now is the warrenty over.
So please Apple be honoust and give warrenty for this problem.