I think that extra month also helps the unknowing to bypass the glitch that SHIII has. The one where, if you change the sub's configuration the game quits tracking your kills.

It's noted to be a SH3 native glitch in the GWX manual... To bypass, you ether scrap the first patrol, or you don't bother with the modification screen until after you come back from the first patrol. Being that you can't kill anything during that month and you're likely not going to change the beginning date the first time with GWX, you bypass the glitch on both fronts.

BTW: My first sub sim was Mircoprose's Silent Service.... on a Tandy 1000, back in '89 or something.