I'm not sure I would be as specific as February :-) But when the forsythia is in bloom in your area is a good rule of thumb or when you see the buds of the new growth begin to swell. Waiting until after frost is really unnecessary. You could even cut them back in the middle of winter if you wanted to.

The leaves of clematis come in pairs on opposite sides of the stem. You want to cut back to just above one of these pairs at an appropriate height. You'll see the buds - even in February they should be obvious and new growth can start forming even earlier in winter. Regardless, the stem will appear to have a narrow ridge at a leaf joint so even without any early budding they should be apparent.

I cut my vines down after they have died back and I am sure they will not start new growth. In Mn this is usually late November.