FYI, 3 hours below zero last night and frost, but it didn't hurt my test Dahlia. Possibility of frost again tonight.
FWIW, its a Lemon Tart cutting that's ~15" tall. Its not near anything artificially heated and wasn't covered.
That is quite surprising. You are indeed very lucky. I'd love to see a photo of the your lemon tart dahlia. Sounds neat.
Here's a picture of the Lemon Tart that handled the frost. Its neat that there's a picture of the Canna in the pond, which did not fair as well as the Dahlia.
Here is a link that might be useful: Lemon Tart beats Canna against the frost