I do mine any time during winter, but I am in a zone that once we get winter, it will be cold for months. Your zone is different, so might need to be cut back in early spring so they don't start really growing if you prune in winter and then get a longer warm spell. At the same time, I wouldn't wait until after spring frosts are done, by which time it would be in full growth. The new leaves are quite frost tolerant IME. I would most likely wait until you see the buds starting to swell in spring as it begins to warm, and then cut it back. Mostly group three clematis are cut back so the flowers emerge on a lower level since they bloom on new wood and to keep the plants looking tidier, so IME you can't do a whole lot of damage, just perhaps delay bloom some.

When does your mystery clematis start blooming? Does it seem to be blooming on old stems for its earliest flowers? Or does it only bloom on new growth? And do you have any photos?

I'll keep an eye on it. The other day it was 80, the next in the 30's, and it froze.

The mystery Clematis I think, blooms on old wood.

This picture was taken right after I got it out of the vine around the first part of April, and if you look closely you can see the old wood. It started blooming shortly after.

Here's a closeup. It bloomed quite a bit last summer. This picture was taken in July.

If it blooms on old wood, I would prune as a type two.

I don't know if this will be helpful to you but I read in some clematis book somewhere eons ago that the best time to prune clems was the same time you prune roses.........or when the forsythia is in bloom (as an indicator plant) and I have always followed that practice. In my climate, that is typically around President's Day or the end of February.

I noticed that several of mine are already pushing a lot of new growth so I need to get out there and get whacking!! I tend also to be flexible about the exact timing. February can be cold in my climate as well and is always wet......and I don't do rain :-)) So I wait until a break in my schedule and a decent weather day coincide and prune as many as I can then.

I also prune both my type 2's and 3's as type 3's - down to the third set of buds or about 18" - simply because it is easier and I don't have to remember who is who :-) At most it will delay the bloom period of the type 2's slightly......a few weeks to a month. All of my evergreen clems or type 1's I don't prune at all except very rarely to control size and that is done after their early bloom cycle.

No offence to Forsythia growers, but that is THE ugliest bush ever, when the blooms are spent. lol. I remember my F. would loose it's beautiful blooms fairly often because of a cold front. That's why it's harder for me to understand why I should prune the Clems when the F. blooms.

Let's take weather out of the equation, should the plants be pruned before or after it starts budding?

My fear is when it looks safe to prune, a freeze shows up a week later.

If the vine is hardy for your zone and established, it doesn't matter if you prune before a freeze. Many clematis growers prune back in late fall. And you can wait until it has budded if you prefer. As I said, many of mine are budded now but I haven't quite gotten to the pruning issue yet.

The point is, there is a lot of flexibility in the timing. And the severity. Pruning too early or too late or too much is really inconsequential to an established vine although it might impact flowering time.

As I said above, you really can't do a whole lot of damage to a type 3 by pruning, either early or late or not at all except perhaps delay blooming, so you don't really need a specific time to prune. I know one grower who prunes her type 3s after their main summer bloom and they put out more growth that reblooms. (She has a longer season that I do.) She also prunes in early spring sometime. It really isn't that crucial. My mom in zone 6 (Cleveland) had one that I pruned for her whenever I visited as long as it wasn't blooming or fully budded. Since I live out of state, that was any time from Thanksgiving to May, and some years it didn't get pruned at all. It bloomed just fine for her.