When should I prune back my roses and dahlias?

Late winter to early spring for roses. Here is a link for dahlias . . .

cit back roses to anout four inches in the fall

When it comes to pruning, I always use the old farmer's method: from Spring thru Fall, only prune during month's that have the letter "R" in their name. For example, you should prune in April but not in May. May does not contain a R. This method is easy for me to remember and I hope it helps you.