The puttees offer a clue to the date.
The website below indicates that the puttees held in place by a strap that wraps around three times, worn by most of the men, is the M1907 type.
A couple of them are wearing a longer type held in place by lace to the side, with a strap under the foot, including, most clearly, the man in the stetson. This may be the one dated 1917, but the website says they are similar to ones used circa 1900, and I can't find a picture of that.
So, the earliest date appears to be 1907.http://www.aef-doughboys.com/leggings.html