WELCOME ABOARD, ryanglavin!
As TarJak said, the whole thing came about because Germany sent 14 u-boats out in advance on August 19, 1939, and a lot of us wished in writing that we could be part of that. With SH3 Commander you can:
1) Go out on a quick pre-war patrol and then set your days in base to 20 or so, and come out again on August 25, which gives you enough time to be on station when the war starts.
2) As was said, just set it to start your career on September 1.
Commander also does a host of other wonderful things you can't get anywhere else. It's the perfect companion piece to GWX3.
