For a Type IX boat, where the patrol technically crosses the year boundary (a December/January patrol, where Jan. is the "P" month) you get to skate by with 1940 modifiers. Close enough for the simpler rule to be appropriate, I felt....so no need to count patrols boxes and split it in half, etc. Just use the starting year for the whole patrol.
However, if you are EXTENDING another month on patrol with any U-boat, and the resupply "P" month is in a new year....you get to use the new year's modifier. Because it is clearly taking place in that new year, if that makes sense.