1917. What I most like about this pattern (after the brilliantined hair, the jaunty pose, and the spats) is the fact that the gentleman is wearing a tie.
This pattern was featured in an illustration in the November 1917 issue of Ladies' Home Journal entitled "Practical Work Clothes and so Easily Made."
This is an interesting style of overalls as there is no waist belt - the bib and trousers are cut in a single length.
Both the fly and the shoulder straps are buttoned.
This is an unprinted pattern.
When I unfolded the pocket piece, I found this fairly substantial thread of fabric. Note that it's plied blue and white. This is not typical of the yarns used to make denim, chambray, or hickory stripe. If used in both the warp and weft, fabric made of this yarn would have had a mid-blue color, somewhat similar to chambray (even though the construction is different.) Because dyeing adds cost, plying dyed and undyed plies will eventually yield an economical fabric. This contributes to an overall sense of the thrift of making work clothes at home.
Updated September 2015 with information from the Ladies Home Journal magazine.