If you are "roaming", maybe if you have your SIM card from another country, this is a common issue for users to experience, having a time from different time zone on a lock screen and a different one in the phone.
Try going to:
Settings>General Management>Date and time, then check if the automatic date and time is on.
If not, turn it on and see what it does.
If it is on, try turning it off and adjust the time zone and the specific time manually, and it will probably get fixed.
Let me know if it worked.
I'm sorry if it didn't work. Btw, if you are a Greek, you can speak Greek.
