Part of the action of the film is supposed to take place after the German invasion of the Netherlands and Belgium, towards the end of May, 1940. However the trees are still without leaves and the actors wear winter clothes.
In an announcement that the Germans troops were near to Paris (June 1940) it is also stated that the French government had decreed the total mobilization of the reserves. Actually the mobilization of the French armed forces had been decreed in September 1939, when the war started.
In announcing the invasion of Germany, it is stated that the German army has invaded Belgium and crossed the French border, occupying the city of Lille. In World War I the German troops crossing into France were advancing first towards the Somme river. It is only later, in their advance towards Dunkirk, that Lille was occupied.