Does the limitation period for a counterclaim run from the same date as the original claim in Ontario?

Not necessarily. A counterclaim, where a defendant being sued brings their own separate claim back against the plaintiff, generally has its own discoverability analysis under Ontario's Limitations Act, 2002, based on when the defendant knew, or reasonably should have known, about their own claim against the plaintiff, which can be an entirely different date than when the plaintiff's original claim was discovered.

In practice, this means a defendant's counterclaim could, in principle, already be time-barred on its own even while the plaintiff's main claim against them is still very much within time, if the underlying events giving rise to the counterclaim happened, and were discoverable, considerably earlier. Conversely, a counterclaim based on more recent conduct could remain fully available even years into a lawsuit over an older dispute. Being sued doesn't itself pause or restart the clock on a defendant's own separate claim; a defendant who has a potential counterclaim should assess its own limitation timeline independently, rather than assuming that simply being an active party to the lawsuit protects a counterclaim that might otherwise already be out of time.