You can either enter a Unix time value (e.g. 496088259) or a date string (e.g. 2009-01-20 08:17:39) into the upper text field OR you can choose a date using the selector below. Choose your time zone and click the 'convert' button.
This depends on the time you want to convert. If you have for example a Unix time stored on a server which runs in Germany with the correct German time set, then you choose GMT +1, because thats the correct time zone for Germany.
Some examples for time zones: