Check the World Clock and let the Time Zone Converter show you the actual time difference. You can choose any location, CET and GMT are just the defaults.

• Central European Standard Time (abbreviated as CET) is 1 hour ahead of GMT. The GMT/UTC offset is GMT plus one hour
• EU Time Zone abbreviations: CET, MEZ (Mitteleuropäische Zeit)
• Like most of Europe, Daylight Saving Time (DST) or Summer Time is observed in CET, where the time is shifted forward by 1 hour. The time difference becomes GMT plus 2 hours
• EU Summer Time Zone abbreviations: CEST (Central European Summer Time), CEDT (Central European Daylight Time), MESZ (Mitteleuropäische Sommerzeit)