From: Santa Rosa, CA
The game should be able to go as late as August 45. However, when playing with East Front off, the game will end on the first May 45 turn (Soviets capture Berlin). With EF on, the game may end before the WA gets to Berlin if the Soviet East Front line advances to the point where Berlin is assumed to have fallen. That's probably what's happening in your games (although it's odd that it's always on May 15). You could attach a save file or email one to firstname.lastname@example.org and I can look and see what's going on. Ideally send me one a turn or two before the game ends.
Info for this and VPs, can be found in section 25.1.2. Part of it reads:
When not using the East Front control option (23.1.1), campaign scenarios will end either immediately when Berlin is captured by the Allies, or at the beginning of the first turn in May 1945 (Berlin is assumed to have been captured by the Soviets by that time). When using the East Front control option (23.1.2), campaign scenarios will end during the Allied turn when Berlin is captured, at the beginning of the turn that Berlin has fallen to the Soviets, or at the beginning of the first turn in August 1945.
If a campaign scenario goes past April 1945, the Western Allies will incur a VP penalty. Starting with the first turn in May 1945, for each additional turn the game lasts, the Allied player will lose 100 victory points.
In addition, the Allies will score 75 points per turn for each turn the game ends early.
Basically the game considers Berlin captured after the historical date as being "late" for VP purposes.
All understanding comes after the fact.
-- Soren Kierkegaard