I forgot to do this myself on my latest GWX 3.0 career start on August 1st '39, but while loading a saved game subsequently I noticed that the GWX folks have suggested a workaround for this on the loading screen - i.e., start your first patrol on 8/1/39, but make it basically a shakedown cruise/trial run that only lasts about 5 days or so - then dock back at base and, with the normal number of days in base setting you will start your next patrol (your first "real" one) just a few days before the war starts - just time to reach your patrol grid and be on station when the shooting starts
Seems like a pretty reasonable approach, since such shakedown cruises were probably pretty common for new skippers/new boats. I'm not sure if there's a way to go back later and rename/renumber some of the saved game files to get rid of that first patrol if you really don't like seeing it in your personnel files later, but there probably is a way to do that if you really want to.