Is it possible to treat a ruptured tendon months later?

Depends on which. A rotator cuff can be fixed late though the results are better if surgery takes place within 3-4 months. Biceps, achilles and patellar tendons should be fixed within 2-3 weeks or often a reconstruction is needed.
Tendon late repair. Late repair can be done of the tendon can be mobilized. Of the joint os supple and f the tissue is of good quality, but often a few mnths later a tednon graft or a tendon turndown or a tendon transfer is needed.