Go to Settings, choose General, and then Date & Time. If option automatically set time and date, set it that way.
Go to Settings, tap on General, tap on Date & Time, and click on Time Zone. Set the right one.
Press and hold the sleep/wake button, and drag the slider to turn off your device. Turn it back on, and check if the time and date are correct.
Check if there is a new version of iOS available and if it is, download and install it.
Back up your files with iCloud. Navigate to Settings, tap on General, tap on Reset, and choose Erase all Content and Settings. Usually, everything will be done in minutes, and you will have to set up your iPhone as new.