Does the limitation period for a third party claim run from the same date as the main claim in Ontario?

Not necessarily. A third party claim, where a defendant being sued brings their own separate claim against someone else, arguing that person should share or bear the responsibility, generally has its own discoverability analysis under Ontario's Limitations Act, 2002, distinct from when the original claimant's claim against the defendant was discovered. The defendant bringing a third party claim generally needs to show when they knew, or reasonably should have known, that the third party bore some responsibility, which can be a later date than when the original claim against the defendant was discovered.

In practice, a defendant often doesn't fully appreciate that someone else should share responsibility until they're actually served with the original lawsuit and start investigating their own defence, which can mean the discoverability clock for the third party claim starts meaningfully later than the clock on the main claim. This distinction matters because it can allow a third party claim to proceed even where, on its own, a direct claim by the original claimant against that same third party might already be time-barred. Because this involves its own separate discoverability analysis, a defendant considering a third party claim should focus on when they specifically became aware of the third party's role, not simply assume the same dates as the main action apply.