As noted above, the internal clock is set to use UTC (which is mostly the same as GMT), and then the operating system is aware of what timezone it is in, and shows the correct time for that timezone, based on an offset from UTC.
You can check and configure which timezone your system is set to, by going to a terminal session and doing:
For example, the UK timezone is set as Europe - London .
Note that you have to use a geographic
timezone in order to get the appropriate Daylight Savings Time adjustments. So right now, in the UK in summer, it is no good just specifying GMT+1 , because that will stay as GMT+1 all year round. You need to specify Europe - London in order for the machine to automatically switch between British Summer TIme and GMT.
Sometimes governments change their mind about timezones. Raspbian (via Debian) gets these updates when you do apt-get dist-upgrade or similar. I recall not too long ago when some South American government changed its mind several times in six months - lots of updates!
Andrew Oakley - Cotswold Raspberry Jam, Gloucestershire - http://www.cotswoldjam.org