was at “virtually the same time” as they discovered their claims
against Dr. Barzo).
 Third, the motion judge made her determination that the
limitation period had already expired in reliance on the broad
allegations contained in the draft amended statement of claim,
and without having identified and considered the appellants’ core
claims against the respondents, and the evidence in relation to
when such claims were or ought reasonably to have been discovered. The motion judge, pointing only to Mr. Morrison’s treatment dates, concluded that the claims against the respondents
ought to have been discovered around the same time the appellants discovered their claims against Dr. Barzo. When the claims
against the respondents are properly considered, however, the
evidence on the motion does not support the conclusion that
the appellants ought reasonably to have discovered all of the
respondents’ potentially negligent acts or omissions before the
third party examinations for discovery.
(1) Where a limitation period may have expired, the Limita-
tions Act establishes the test for adding parties
 Subrule 5.04(2) of the Rules of Civil Procedure, R.R.O.
1990, Reg. 194 provides for the addition of parties during
5.04(2) At any stage of a proceeding the court may by order add, delete or
substitute a party or correct the name of a party incorrectly named, on such
terms as are just, unless prejudice would result that could not be compensated for by costs or an adjournment.
 At the same time, s. 21(1) of the Limitations Act prohibits
the addition of a party to a proceeding where the limitation
period has expired:
21(1) If a limitation period in respect of a claim against a person has
expired, the claim shall not be pursued by adding the person as a party to any
 In Arcari v. Dawson (2016), 134 O.R. (3d) 36,  O.J.
No. 5069, 2016 ONCA 715, at para. 7, leave to appeal to S.C.C.
refused  S.C.C.A. No. 522, this court observed that the
effect of s. 21(1) is that “the clear expiration of a limitation period
is an absolute bar to the addition of a party to an already existing
 Under the current Limitations Act, the date of “discovery”
is key to assessing whether the limitation period in respect of
a claim has expired. Sections 4 and 5 provide as follows: