There is no " cut off point" as such........last year one or two dahlias were,very late in showing growth. This year some are shooting and others aren't.
However, I assume your dahlias in pots were,left outside too. It's a difficult winter to say if dahlias will survive because of the non stop rain. I'm in a very mild area where pelargoniums and gazanias are still,flowering but dahlias with their tubers buried may have rotted this year. I dont know yet. So, I would say mid April we should see dahlias shooting. I left begonia corms outside in pots and will check on those later.
Although its been a wet winter it hasn't been cold so maybe these things will be fine.
(just checked my begonia corms and they are solid and not rotten so hopefully dahlias.....both yours and mine....will be ok too)