Can you heal a ruptured tendon months later?

Yes: You could but would depend on the exact type, extent and location of injury. For example if you completely rupture a flexor tendon to one of your fingers and it completely retracts into the palm or wrist which it often does, there is no way for it to heal. The patient might adapt but that's a different story. In contrast, for example, a ruptured extensor tendon, so-called mallet finger, could heal.

Depends: Tennis elbow is a kind of tendon rupture in some people and this can heal with time. However a finger tendon which is cut or ruptures will not. Are you refering to some specific tendon?

Bad prognosis: A truly ruptured tendon that has not been treated and whose ends are separated is unlikely to heal spontaneously and is likely to require surgery and/or a graft