First clue is the rifle- a Springfield Model 1903. So it's after 1903 and before the First World War in 1917. They're wearing the army blue dress uniform which wasn't really issued during the war. The cap insignia badges actually look like the crossed cannon of artillery. No overseas stripes or medal ribbons, the man in the upper left appears to have a Spanish American War medal. So you're looking at roughly 1904- 1916, I'd say closer to the lower end of those dates.