Some flower on old wood, some on new wood, and others on both old and new wood. It’s best to prune White Diamonds in early spring before it starts putting on new growth. It flowers on new wood. If you wait too long in the spring to prune, you run the risk of cutting off the flower buds. I would only prune it in the fall if there is a broken branch or a branch that it driving you crazy.
Thanks for writing!