What's the difference between Ontario's two-year limitation period and the fifteen-year ultimate limitation period?

Ontario's Limitations Act, 2002 sets out two different clocks that can apply to the same claim. The basic two-year limitation period runs from the day the claim was discovered, meaning when the claimant knew, or reasonably should have known, about the loss and who caused it — this is the clock most people think of when they picture "the limitation period." The fifteen-year ultimate limitation period runs independently from the day the underlying act or omission actually happened, regardless of when, or whether, it was ever discovered.

The ultimate period exists as an outer boundary specifically for situations where discoverability could otherwise let a claim stay alive indefinitely, sometimes decades after the events in question, simply because a claimant genuinely didn't know about the harm sooner. In most everyday claims, the two-year discoverability clock expires first, so the fifteen-year period rarely becomes the operative deadline. It matters most in cases involving harm that surfaces very slowly, such as certain long-term property or environmental issues, where discovery might otherwise be delayed well beyond fifteen years from when the conduct actually occurred. Once fifteen years have passed from the act or omission, the claim is generally barred regardless of when it was discovered, with narrow exceptions the Act itself sets out.