Daylite is indeed the trigger (or lack there of .) In the spring when there is a coresponding measure of daylite , often elk tend to then bugle also.I believe there may still be more light in Northern BC at the beginingof Sept but need to check on this. The rut always starts about the same time. What changes is the level of activity. Because you don't hear a buggle does not mean the elk aren't bein social, just low key.
The whitetail and mulies rut here mid Nov, not sure when in Colorado.