It's almost working perfectly, or at least the setup is. I got into the game to choose the career and it believes I've chosen a Type XXI. (That'd be nice, but I'm waiting on one of those...)
I had just finished playing the
Bismarck single mission with a XXI, so that may be influencing it. Here's what Patrols_0.cfg has to say:
PatrolNumber=0
StartYear=1941
StartMonth=3
StartDay=22
StartHour=2
StartMin=22
EndYear=0
EndMonth=0
EndDay=0
EndHour=0
EndMin=0
RealYear=2010
RealMonth=4
RealDay=19
RealHour=23
RealMin=14
StartLocation=753
EndLocation=753
SubType=2
SubVersion=1
SubName=U-126
PatrolArea=BE29
That's the relevant data for the file, and 22 March 1941, oddly enough, was the
real first sailing of U-126, so I made the change. The bracket at the top of the data is Patrol 010, for what that's worth.
NB: I just tested it in-game and the bloody thing is insisting that the date is 20 December 1940. I give up! I was going to have the crew home for Christmas, so there's going to be an awful incident of sabotage aboard my IXB, or something, a mile or two out of the pen.

Bloody shame about my time aboard U-122. Really.