If you change the timezone (dashboard - preference) to another default it will not be restored at a next login.
So you have to change it again... and again ..and ;-)

in my/config.php find the variable $timezone and edit to suit your timezone. This will change the timezone for the whole site

I know, but don't know what to fill in.
I've tried GMT+1 but that doesn't work.
Do you?

if you want gmt +1, change to:

$timezone = +1;

this will work if your server time is set to gmt. If not, you will need to know which timezone your server is in, eg. my host is in USA so I need to use:

$timezone = -8;

to get the correct GMT time with daylight saving