There is no way to date this photo with complete accuracy, but I can tell you some things based on the clothing your great grandparents are wearing that should help narrow the date for you.

This photo is not from the 30's.

Although the hat your great grandmother is wearing compares to some styles worn at the very end of 1930 ... it even more closely resembles the styles worn through the 1940's. The Homburg style was popular in the 1940's for both men and women due to Winston Churchill's influences.

The hat your great grandfather is wearing was indeed in style during the 1930's as well as through the 1940's.. however the fedora was not worn at an angle with the back brim facing upward as your great grandfather is wearing his until the early 1940's. (The "Film-Noir" effect of fashion for men's hats)

The bold flowing design of your great grandfathers tie first became fashionable around 1943 till the early 1950's. This type of tie became mainstream for men around 1945 at the end of the war, popularized by movie stars such as Danny Kaye and Bob Hope. By the 1950's, the ties started getting less wide than this one appears.

Now.. a clue that you can use to help date this more accurately is the boutonniere that your great grandfather is wearing. During WWII and for a few years on, there was a wartime shortage of flowers. It was unusual for a working man to wear a lapel flower during the early 1940's because of this. To me this indicates a special event, such as a wedding. Checking newspaper archives for wedding announcements for your family during the 1940's just might give you an exact date for this picture. I suggest concentrating the search between 1942 and 1948.