17. The audience knows exactly when the events of the film take place, if it cares to look
The Wizard of Oz was set in the depression, with Dorothy Gale coming home from school one afternoon (presumably, since she is carrying books) when her dog damages Almira Gulch’s flowers. But there is nothing in the early scenes in Kansas to indicate what year it is, or what time of year. Once the twister lifts the farmhouse from its foundations and carries it to Oz, there is no indication of which direction it traveled, nor whether the journey is to another time, in the past or in the future.
Upon arrival in Oz and Dorothy’s encountering Glinda, followed by her meeting the tittering Munchkins, the date is revealed. The Coroner steps forward to examine the late Wicked Witch of the East and declares her dead. He has a Death Certificate for the Witch, including a date, which reads May 6, 1939, from which it can be assumed that the residents of Oz used the same calendar as those of Kansas. The date is not a coincidence, it was chosen deliberately because it was the 20th anniversary of the death of L. Frank Baum.