Your mission will have a start date. You can find this by going to the "Mission-->Parameters" menu. This is the date/time when the mission starts.
The IXC has an entry date. This is the date at which this class of U-Boat was introduced into service. You can find this by clicking on the unit to highlight it, then right click and select properties. Look for "entry date", about half way down.
You can change the entry date within certain limits. Otherwise you will need to alter the date in which your mission occurs.
I hope that helps!
