With temperatures reaching 76 today, many plants are still in bloom through Indian Summer. Do not dig your dahlias and other tropical bulbs too early. Wait until 2 weeks after a killing frost. If you have not had a frost by November 15, then it is safe to dig at that time. The tubers need time to harden off and prepare for winter storage. If you dig too early they will be "green" and will not store properly and may not survive storage.
Can your dahlias survive the winter outdoors? Probably not. But I have one dahlia that has survived five Zone 6 winters in the ground. It enjoys optimal conditions, however, planted next to my heat retaining granite foundation on the south side of my house on top of a slope. Just remember leaving dahlias in the ground over the winter is never a guarantee. We are all at Mother Nature's mercy.