From Toronto in mid September to Quebec City in late September, hypothetically you really could encounter everything from shorts-and-sunscreen weather to boots-and-jacket weather.
The driving distance between Denver, CO and Salt Lake City, UT is 536 miles. The driving time would be approximately 8 hours 30 minutes if driving non-stop in good conditions.
From there it is an 8.5-9 hour drive to the Yellowstone area. Salt Lake is a 5+ hour drive. Just wanted to add Idaho Falls Airport, which puts you 90 minutes for Jackson and just under 2 hours to West Yellowstone. We are also served by the hubs in Seattle, Denver, Minniapollis, and Salt Lake City.