Pearl Harbor has been attacked (7 December 1941), and Washington D.C. is in full wartime mode (early 1940's) when Mrs. Marshall falls ill; bedridden for almost three years, she looks over a stack of checks that have gone to pay her medical bills; they are all dated March, April, and June 1937.
Peter Marshall's son, Peter John Marshall, was actually born on January 21, 1940, not December 7, 1941, as depicted in the film. See http://petermarshallministries.com
The bulk of the story takes place in the 1930's and the 1940's but all of the women's fashions and hairstyles are strictly 1955.