I have done several experiments KaleunMarco, and like you, have come to the conclusion that the game ENDS on a certain date, come H-E-double-hockey-stix or high water, and THOU SHALT NOT DEVIATE from it...
If you try to "end" earlier, you will be doing Groundhog Day until August 1945, or you will have to abandon the scenario... In SH3, it is May... "Surrender this day, else perish!"
So it is either
-OR-
... frustrating