A wildlife biologist I talked to once, said the amount of light entering the cow elk’s pupil is what triggers the rut.
But not all cows come into heat at the same time, older cows usually start first, younger cows a bit later.
Cows are triggered to come into estrus (and cause the peak rut period) within 5-10 days of the Autumn equinox,
which is when the day and night are of equal duration.
This year (2017) the fall equinox will be Sept. 22 here in the northern hemisphere.
Many other factors can and will affect the elk rut - weather, hunting pressure, and the presence of wolves.
One major factor that effects elk and deer movement is heat - early in the season when it can still be hot outside, depending on elevation,
most animals won't be too far from water, and they won't be moving around much.
If you can catch an elk or a deer heading to the river in the evening before dark, or moving off the river just after dawn, you might be in luck.
But that type of success can depend on how much rain fell that year, lots of rain might mean animals won't have to travel far to drink.
Hot Indian Summer days (especially these last few years with climate change) along with a full moon can negatively affect conventional "dawn and dusk" elk hunting.
Where I work, at one of the mines up in Elkford, every September I hear the bulls bugle a good week or so earlier than where I live here around Cranbrook, which is about 1200 feet lower in elevation, but only about a 2 hrs drive south.
Another big factor is the wolf, in areas where, and during times when wolves are in overpopulation, bull elk seem to bugle less, as they know it will alert the wolves to their location.
The last couple of seasons, as soon as it gets dark, I hear the wolves howl, but hardly an Elk bugle.