Using buttons near to time display, Press "set"" several times until the year is flashing.
Then press and hold "set" for 3 seconds.
"Rds time 1" comes up on the screen
Change the 1 to 0.
Then press set again.
This will disable the RDS sync. This should fix your problem and stop the time changing itself after you set the right time and date manually.
Rupert
April 2020