Charles Fraser

Correction: History seems to be different in this movie. It's possible that the second world war is going on during this movie or has already happened before it. At one point in the movie somebody mentions that the last notes by Totenkopf were "written in 1918, over twenty years ago". The key word being "over". Also, the Hindenburg III docks with the Empire State Building in the beginning, which it was actually designed for except it never happened because of the high winds. The original Hindenburg was destroyed in 1937. So another would have had to have been built and either lost, destroyed or retired and then a third built. This would take some time. There's also a wreck underwater that I believe is supposed to be the Titanic and it's in one piece. They seem to have created their own history loosely based on our own.

Charles Fraser

Correction: Another few time-stamps to note are the screening of the Wizard of Oz which was first screened on 1939. I'm not sure how things went during that period, like how many years movies ran for. Wikipedia does mention a re-release in 1949 to re-attempt to cover costs. The second time stamp is vehicle design. The cars featured do not seem to yet feature the ponton style of integrating fenders into the car's body, though it would appear the fenders of this age were already getting rounder and more integral and had lost running boards a la VW Beetle. Again, this isn't too concrete since I don't suppose people threw their cars away the moment the new style came up. The third time-stamp would be the mention of global forces being spread thin and their dependence on the Sky Captains as an established mercenary company. Additionally, fabric rationing also placed women's skirt hemlines just below the knee during the 1940s as worn by Polly. This could perhaps place the movie in the 1940s.