a person is presumed to have actually known of the matters
referred to in s. 5(1)(a) on the date the act or omission on which
the claim is based took place. However, the presumption of actual
knowledge established by s. 5(2) can be rebutted by proof to
 As was emphasized above, the presumption in s. 5(2) only
operates in relation to the determination of a claimant’s actual
knowledge for purposes of s. 5(1)(a). Even if a claimant can show
that they did not actually know of the elements of their claim,
thus rebutting the presumption in s. 5(2), their claim might still
be barred on the basis that a person with their abilities and in
their circumstances ought to have known of those matters more
than two years previously.
 In addition to the basic two-year limitation period established by ss. 4 and 5, s. 15(2) provides an ultimate limitation
period of 15 years following the “day on which the act or omission
on which the claim is based took place”. Significantly, this language in s. 15(2) is identical to that utilized in s. 5(2) although,
unlike s. 5(2), the ultimate limitation period in s. 15 cannot be
rebutted or displaced by evidence to the contrary. The 15-year
ultimate limitation period applies even if a limitation period
established by any other section of the Act has not expired.
 Section 18 establishes a special rule for the determination
of the limitation period for claims for contribution and indemnity
brought by one alleged wrongdoer against another. As was just
pointed out, both ss. 5(2) and 15(2) make reference to “the day on
which the act or omission on which the claim is based took place”.
What s. 18 provides is that, in cases of claims for contribution and
indemnity brought by one alleged wrongdoer against another, the
day on which the act or omission on which the claim for contribution and indemnity is based shall be “deemed” to be the day on
which a claim is served on the first alleged wrongdoer.
 Two important aspects of s. 18 are worth noting. First,
s. 18 is merely an interpretive provision that does not itself purport to extinguish or bar claims. Rather, it merely assists in the
interpretation and application of ss. 5(2) and 15 which, in combination with ss. 4 and 5(1), are substantive provisions setting out
time limits within which claims must be commenced.
 Second, s. 18 does not specify the date upon which a claim
is “discovered”. It merely specifies the date upon which “the act
or omission on which the claim is based took place”. This date is
certainly a key element or consideration in determining the day
on which a claim is discovered, but the discovery of a claim is governed by the operation of s. 5(1)(a) and (b).