Yes, I noticed it, too. The next day after you arrive in port you are in the mission setup room, but when you start your next mission, the random amount of time is added to the date. So when you start a mission and are in port, look at the date in lower right corner, and you'll see that it is some days later depending on the year.
The most bugging thing is that you can only use the tech of the day that you last entered port, and your next mission date might be more then a month later if you play in 1944-45. Also, I wonder if this might be a problem if you for example enter port on say May 1st, 1945, the ready room then displays the 2nd of May, but when you start a new mission: blam, it's the 13th of June 1945, the war is over, and there you are in your U-boat, ready to sink...dolphines.
