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If your phone is Android-type, then you may need to either
A. set the time correctly again, manually,
B. set the time to be updated according to the time available over the network.
For B. try Settings - Date/Time - Automatically update (enable that option.)
Or on some phones,
Settings - System - Date and Time - Automatic date and time
If Automatically Update is already enabled, then disable it... the network source by which it is being updated is probably what is losing time, in that case.
For A. it's much the same, only you have to disable/uncheck the Automatic setting, to use the Set Time options to set the time.