Time in SAP

Hi,

In my SAP I have system time with time zone GMTUK and SAP system time with CET (installation default).

The correct one is GMTUK.

The users never found out because they are created with WET time zone that is equal to GMUK...

I know that I should chage SAP system time to GMTUK, but can anyone tell me the implications of the system running as is vs change it.

Best Regards