iPhone5S clock stops working

iphone 5S model A1529 operating system 8.1.3

Yesterday, the clock of my phone stopped working when the phone goes in its cover / lock mode.
Whatsapp and SMS's give the correct time for incoming messages and the (wrong) phone time on outgoing ones.
Alarm clock obviously unreliable as time incorrect.
I have gone through questions on the forum and have tried what was mentioned in iOS: Troubleshooting issues with date and time - Apple Support. Also tried other advice from Sterling r on April 16, 2014 from question "discussions.apple.com/thread/6090339"

Please assist. Thanks.