If its an Android OS Phone, it might be using Automatic date & time, it uses network-provided time or GPS-provided time. If you want to see or modify this go to Settings > System > Date & Time > Automatic date & time
There you can find options like
-
Use netnetwork-provided time
-
Use GPS-provided time
-
Off
For iPhone:
Go to Settings > General > Date & time
Then select Set Automatically
