WELCOME ABOARD!
Yes indeed, GWX starts your career a month before the war does. This gives you time to get the feel of things as well as be on station already when the war begins. They sent 14 u-boats out to the Atlantic on August 19 for just that reason.
I start my first patrol and immediately end it, then use SH3 Commander to set my days in base to 18, and then leave on the 19th for the beginning of the war. Works every time.