SET TIMEZONE doesn't affect to SELECT statement

[POST <post@xxxxxxxxxx>]

Title: SET TIMEZONE doesn't affect to SELECT statement

Hello; by tutorial I have tried the example:

set timezone to 'Europe/London';

select '2008-01-01 12:00:00 GMT+2'::timestamptz;

Result: 2008-01-01 15:00:00

set timezone to 'Europe/Moscow';

select '2008-01-01 12:00:00 GMT+2'::timestamptz;

Result: 2008-01-01 15:00:00

Why is the result identical ? Is something wrong ?

Thanks for suggestion.