I have an iphone 5C and the date/time keeps changing all the time, even if I change it mannualy or select automaticaly, the date/time changes a few minutes later for a totally wrong time, the wrong hour doesnt even passes by! Help pls!

I have an iphone 5C and the date/time keeps changing all the time, even if I change it mannualy or select automaticaly, the date/time changes a few minutes later for a totally wrong time, the wrong hour doesnt even passes by! Also, the phone is operating slowly (this was before the IOS 8 and after is even slower) sometimes I open the camera and it doesnt respond to my command. It's a brand new phone and I dont understand... my iphone 4S is faster and is working perfectly compared to this 5C.

Thanks

Sarah