[154]*154LEESON, J.
This case is before the court on certified questions of Oregon law from the United States Court of Appeals for the Ninth Circuit under ORS 28.200 et seq. and ORAP 12.20. We take the following facts from the Ninth Circuit’s certification order:
“Shasta View Irrigation District (‘Shasta’) was organized and formed on December 5,1917, pursuant to Chapter 357 of the Oregon Laws of 1917, now Chapter 545 of the Oregon Revised Statutes. There are approximately fifty-six individual members of Shasta. These members own irrigable land within the geographic boundaries of Shasta, and farm or lease their land to others.
“Under a rehabilitation and betterment project proposed by Shasta, over twenty-one miles of existing unlined canals were to be replaced with approximately seventeen miles of buried pressure pipeline. In April 1972, Shasta entered into a contract with the United States Bureau of Reclamation. The contract provided that the United States would loan up to $3,222 million to Shasta for the rehabilitation and betterment of the irrigation system. The term of the loan was sixty-five years.
“In April 1973, Shasta released the bid specifications for the project. The specifications provided, among other things, that the pressure pipe used in constructing the irrigation system must last for the sixty-five year term of the loan.
“In June 1973, Shasta entered into a contract for construction of the pressurized irrigation system. The pipe installation contractor elected to use two different types of pressure pipe in constructing the system: Techite, a brand of reinforced plastic mortar pipe manufactured by Amoco Reinforced Plastics Co. [Amoco] was used in approximately 50,000 feet of the system and an asbestos-cement pipe manufactured by Certainteed Products Corp. was used in approximately 38,000 feet of the system.
“The Techite was ordered from Amoco * * * between June and September 1973. Installation of the pipe began in February 1974 and was completed by June of that year. In July 1975, Shasta accepted the irrigation system as complete and operational.
[155]*155“According to Shasta, the Techite pipe has failed approximately twenty-six times, beginning in 1978, with two of the failures occurring before July 1,1982. On February 16, 1989, Shasta’s board of directors voted in favor of pursuing legal action against the manufacturer of the Techite pipe. In 1994, Shasta filed the present action in Oregon superior court [sic], alleging strict product liability, negligence, breach of express warranty and fraud/misrepresentation.
“After removing the action to federal district court on the basis of diversity of citizenship, Amoco filed a motion for summary judgment, claiming that all of Shasta’s claims were statutorily time barred. The district court granted summary judgment in favor of Amoco, finding that Shasta was not a ‘public corporation’ for purposes of statutory time limitations and that, therefore, all of Shasta’s claims were time barred by Oregon’s Product Liability statutes. [Shasta] timely appealed.”
The Ninth Circuit certified the following two questions to this Court:
“1. Is an irrigation district a ‘public corporation’ for purposes of applying the exemption to applicable limitations set out in [ORS] 12.250? If an irrigation district is a ‘public corporation’ under [ORS] 12.250, then:
“2. Does [ORS] 12.250’s exemption to applicable limitations apply to [ORS] 30.905(1), a statute of ultimate repose outside of [ORS] chapter 12?”
We accepted certification of those questions and, at that time, added an additional question:
3. “If ORS 12.250 does not apply to ORS 30.905(1), then is there a common-law variation of ORS 12.250 that would apply to ORS 30.905(1) to make Shasta’s action timely?”
See Western Helicopter Services v. Rogerson Aircraft, 311 Or 361, 370-71, 811 P2d 627 (1991) (court has discretion to reframe questions presented). We address the questions in order.
[156]*156CERTIFIED QUESTION NO. 1:
“Is an irrigation district a ‘public corporation’ for purposes of applying the exemption to applicable limitations set out in [ORS] 12.250?”
ORS 12.250 provides:
“Unless otherwise made applicable thereto, the limitations prescribed in this chapter shall not apply to actions brought in the name of the state, or any county, or other public corporation therein, or for its benefit.”
(Emphasis added.) Shasta contends that the legislature has recognized the public nature of irrigation districts and that several statutes “confirm the general proposition that Oregon law treats irrigation districts like other [public] corporations.” Furthermore, Shasta argues, there is no evidence in ORS 12.250 that the legislature intended not to extend the protection of ORS 12.250 to irrigation districts. Amoco replies that the text and context of ORS 12.250 leads to the “inescapable conclusion that irrigation districts are not one of the ‘other public corporation[s]’ entitled to an exemption within the meaning of that statute.”
To answer the first certified question, we must construe the phrase, “other public corporation therein.”1 The starting point of our analysis is the text and context of the statute, giving words of common usage their plain, natural, and ordinary meaning. See PGE v. Bureau of Labor and [157]*157Industries, 317 Or 606, 610-11, 859 P2d 1143 (1993) (describing statutory construction methodology).
The term “public corporation” is not a term of common usage, and neither the statute nor ORS chapter 12 defines the term. However, this court has held that a public corporation is a corporation formed for the public’s benefit or for a public purpose. See State ex rel Eckles v. Woolley, 302 Or 37, 48-49, 726 P2d 918 (1986) (so stating); see also Black’s Law Dictionary, 1228 (6th ed 1990) (defining public corporation as a municipality or government corporation “created for the administration of public affairs”). This court also has held that “[t]he concept of a ‘public corporation’ covers a wide variety of institutions,” including municipal corporations. Eckles, 302 Or at 47.
Two words of common usage in ORS 12.250 modify the term “public corporation”: “other” and “therein.” Based on the express words of the statute, we conclude that the exemption described in ORS 12.250 applies to corporations in addition to — or other than — the state and counties that are formed for the public’s benefit.
The next inquiry is whether an irrigation district is formed for the public’s benefit or for a public purpose. An irrigation district is a corporation formed to foster the beneficial use of water by the public. See ORS 545.249 (irrigation district’s use of all water, water rights, and rights to appropriate water “declared to be a public use more necessary and more beneficial than any other use”). In Twohy Bros. Co. v. Ochoco Irr. Dist. et al, 108 Or 1, 11, 216 P 189 (1923), this court held that an irrigation district is a municipal corporation, because “its property [is] public property and its officers [are] public officers, elected by the legal voters of the irrigation district * * *. Such a district ‘is created for a public purpose ***.’” From the foregoing, we conclude that an irrigation district formed under ORS chapter 545 is a public corporation within the meaning of ORS 12.250.
We answer Certified Question No. 1 “YES.”
[158]*158CERTIFIED QUESTION NO. 2:
“Does [ORS] 12.250’s exemption to applicable limitations apply to [ORS] 30.905(1), a statute of ultimate repose outside of [ORS] chapter 12?”
The second certified question subsumes two issues. The first is whether the exemption in ORS 12.250 is restricted to limitations prescribed in ORS chapter 12, or whether the exemption applies outside ORS chapter 12 as well. The second issue is whether the exemption applies to ORS 30.905(1), which is a statute of ultimate repose outside ORS chapter 12.
Shasta argues that the exemption from applicable statutes of limitations applies outside ORS chapter 12, because ORS 12.250 contains a policy reflected in the Latin maxim, “nullum tempus occurrit regi,” which means that “[t]ime does not run against the [sovereign].” Black’s Law Dictionary at 1068. Amoco responds that resolution of the issues is a matter of statutory construction. We agree. Consequently, we turn again to the template for statutory construction, looking first to the text and context of the statute. PGE, 317 Or at 610.
ORS 12.250, quoted above, consists of a single sentence. The exemption contained in that statute unambiguously refers to “the limitations prescribed in this chapter.” See Chizek v. Port of Newport, 252 Or 570, 577, 450 P2d 749 (1969) (exemption in ORS 12.250 does not apply outside ORS chapter 12). Consequently, unless some other statute were to make the exemption contained in ORS 12.250 applicable to a proceeding outside ORS chapter 12, that exemption applies only to the limitations contained in ORS chapter 12.
We next inquire whether ORS 30.905(1) itself makes the exemption in ORS 12.250 applicable. ORS 30.905(1) provides:
“Notwithstanding ORS 12.115 or 12.140 and except as provided in subsection (2) of this section and ORS 30.907 and 30.908(1) to (4), a product liability civil action shall be commenced not later than eight years after the date on which the product was first purchased for use or consumption.”
[159]*159By its express terras, ORS 30.905(1) does not make the exemption in ORS 12.250 applicable to the ultimate repose period for commencing a products liability civil action. The only reference in ORS 30.905(1) to limitation periods contained in ORS chapter 12 is that the ultimate repose period in ORS 30.905(1) applies notwithstanding — that is, in spite of — the limitations contained in ORS 12.1151 2 and ORS 12.140.3 Thus, the text indicates that the legislature did not intend that the exemption in ORS 12.250 apply to the ultimate repose period in ORS 30.905(1).
We conclude that the exemption granted to the state, counties and other public corporations by ORS 12.250 does not apply to the statute of ultimate repose in ORS 30.905(1).
We answer Certified Question No. 2 “NO.”
ADDITIONAL QUESTION NO. 3:
“If ORS 12.250 does not apply to ORS 30.905(1), then is there a common-law variation of ORS 12.250 that would apply to ORS 30.905(1) to make Shasta’s action timely?”
We note at the outset that the question is not whether there are any common-law rules that might be relevant to the analysis in this case. Rather, the question is whether there is a common-law variation of ORS 12.250 that might be relevant in analyzing Shasta’s claim. There is.
The common-law variation of ORS 12.250 is that general statutes of limitations do not run against the government unless the statute “otherwise expressly provide[d].” State v. Warner Valley Stock Co., 56 Or 283, 308, 108 P 861 (1910); see also Corvallis Sand & Gravel v. Land Board, 250 Or 319, 338, 439 P2d 575 (1968) (stating same rule). In State [160]*160Land Board v. Lee, 84 Or 431, 434, 165 P 372 (1917), this court explained:
“Th[e] rule is said to be founded upon the legal fiction expressed in the maxim nullum tempus occurrit regi. However, it is not necessary to predicate this salutary precept upon any fiction, since sound reason for the rule is found in the fact that as a matter of public policy it is necessary to preserve public rights, revenues and property from injury and loss by the negligence of public officers * *
Oregon originally recognized governmental exemption from general statutes of limitations afforded by the common law. State Land Board, 84 Or at 435. However, in 1862, the legislature abolished the rule by enacting Section 13, Deady’s Code, which provided:
“The limitations prescribed in this title shall apply to actions brought in the name of the state, any county or other public corporation therein, or for its benefit, in the same manner as to actions by private parties.”
General Laws of Oregon, ch 1, § 13, p 142 (Deady 1845-1864) (emphasis added). Under Section 13, the state, counties, or other public corporations lost their common-law exemption from the running of general statutes of limitations. In 1903, the legislature restored that exemption from the running of most statutes of limitations by amending Section 13 as follows:
“The limitation prescribed in this title shall not apply to actions brought in the name of the state, or any county, or other public corporation therein, or for its benefit * *
Lord’s Oregon Law, v I, title I, ch II, § 13, p 140 (1903) (emphasis added). In other words, between 1862 and 1903, the legislature expressly consented to the application of statutes of limitations against the state, any county, or other public corporation. However, in 1903, the legislature revived and codified a version of the common-law rule that general statutes of limitations do not apply to those entities, unless the statute expressly or by necessary implication provides otherwise. State Land Board, 84 Or at 436.
With the exception of the introductory clause, which was added in 1953, the text of section 13 has remained [161]*161unchanged since its amendment in 1903. That exemption from general statutes of limitations for the state, counties, or other public corporations currently is codified at ORS 12.250.
As explained in the analysis of the second certified question, the exemption in ORS 12.250 is limited to statutes of limitations contained in ORS chapter 12, unless the legislature expressly makes ORS 12.250 applicable outside ORS chapter 12. Nonetheless, the reasoning behind the common-law exemption has guided this court in determining whether a statute of limitations outside ORS chapter 12 bars an action brought on behalf of the state, county, or other public corporation. For example, in State Land Board, the question was whether the running of a ten-year statute of limitations in General Laws of Oregon 1913, chapter 304, barred the State Land Board from foreclosing on a mortgage lien. This court held that the statute did not bar the State Land Board’s action, because the court viewed the statute “in the light of the previously declared policy of the state,” id. at 436, namely, that “it is necessary to preserve public rights, revenues and property from injury and loss by the negligence of public officers.” Id. at 434; accord Chizek, 252 Or at 578 (public would not be served if claim of Port of Newport, as a public body, were barred by statute of limitations in ORS 312.230); cf. Withers et al v. Reed, 194 Or 541, 556-57, 243 P2d 283 (1952) (forfeiture statute applies against state, because common-law exemption from limitation periods is not applicable where negligence of public officials harms public good).
With that background, we turn to whether the common-law exemption applies to exempt Shasta from the operation of ORS 30.905(1). To resolve that issue, we must analyze the purposes underlying statutes of limitations and statutes of ultimate repose.
Statutes of limitations “limit[ ] the time a party has to initiate an action once a claim has accrued.” Sealey v. Hicks, 309 Or 387, 394 n 7, 788 P2d 435 (1990); see also Baker v. Kennedy, 317 Or 372, 376, 856 P2d 314 (1993) (“Statutes of limitation refer to commencement of legal actions.”) (emphasis in original). Generally, a statute of limitations does not begin to run until the injured party knows or should know that it has been injured. Gaston v. Parsons, 318 Or 247, 259, [162]*162864 P2d 1319 (1994). But see Huff v. Great Western Seed Co., 322 Or 457, 464, 909 P2d 858 (1996) (plaintiff’s belated discovery of employer’s unlawful motive does not delay commencement of statutory limitations period in case brought under ORS 659.121(1)). The expiration of a statute of limitations is an affirmative defense to an action. ORCP19 B. However, statutes of limitations can be waived, Multnomah County v. Dept. of Rev., 325 Or 230, 234, 935 P2d 426 (1997), and various circumstances can toll their expiration. See, e.g., Baker, 317 Or at 374 (statute is tolled if insurer fails to give injured party timely notice of running of statute of limitations after making advance payment for damages). In sum, although statutes of limitations may affect a plaintiffs ability to enforce a remedy, the running of such statutes does not extinguish a right. See Evans v. Finley, 166 Or 227, 233, 111 P2d 833 (1941) (so stating); Goodwin v. Morris, 9 Or 322, 324 (1881) (same).
The legislature enacts statutes of ultimate repose to supplement applicable statutes of limitations. DeLay v. Marathon LeTourneau Sales, 291 Or 310, 315, 630 P2d 836 (1981). The legislature enacted Oregon’s first statute of ultimate repose in 1967. Id. Statutes of ultimate repose set maximum times to file a claim, regardless of the date of discovery of an injury or other circumstances that may affect the expiration of a statute of limitations. See Josephs v. Burns & Bear, 260 Or 493, 498, 491 P2d 203 (1971) (statute of ultimate repose in ORS 12.155(1) intended to provide overall maximum time limitation); see also DeLay, 291 Or at 314-15 (medical malpractice statute of ultimate repose not tolled by the plaintiffs insanity). An ultimate repose period “provides a deadline for the initiation of an action whether or not the injury has been discovered or has even occurred.” Sealey, 309 Or at 394 n 7. Unless otherwise provided by statute, an ultimate repose period “cannot be extended regardless of unfairness to the plaintiff.” DeLay, 291 Or at 315. Once an ultimate repose period has expired, the claim is extinguished and no legally cognizable injury exists. See Sealey, 309 Or at 392 (so declaring with respect to products liability statute of ultimate repose).
The legislature enacted ORS 30.905(1) in 1977. This court previously has explained that
[163]*163“ORS 30.905 was the result of a major 1977 lobbying effort by business and insurance organizations for reform of the common law of products liability. The perceived problem was the high cost of hability insurance. One of the legislative solutions was to fix a limited and predictable time period in which a manufacturer, distributor, seller or lessor would be exposed to a product liability civil action.”
Erickson Air-Crane Co. v. United Tech. Corp., 303 Or 281, 286, 735 P2d 614 (1987). This court further explained that the assumption throughout the legislative consideration of the statute was that “manufacturers, distributors, sellers and lessors should have the benefit of a limited and predictable time period during which they would be exposed to liability for defects that existed when the product left a respective party’s hands.” Id. at 288. ORS 30.905(1) establishes that time period as eight years.
In Sealey, this court held that ORS 30.905(1) reflects the legislature’s determination “that an injury occurring more than eight years after a defective product first entered the stream of commerce is not legally cognizable.” 309 Or at 392 (emphasis added). The court also declared that enactment of ORS 30.905(1) was within the legislature’s authority “to determine what constitutes a legally cognizable injury.” Id. at 394.
As discussed above, the court explained in State Land Board the common-law rule that the government is not included in general statutes of limitations, unless included expressly or by necessary implication. 84 Or at 434. The public policy reason for the exemption is to protect the public from the negligence of public officials who fail to assert claims in a timely manner after they discover, or should have discovered, an injury. Id. Consistent with that policy, the exemption means that governments’ ability to enforce legal remedies is not affected if public officials are negligent in failing to assert claims.
ORS 30.905(1), by contrast, reflects a legislative judgment that an injury occurring eight years after a defective product first enters the stream of commerce is not legally [164]*164cognizable because, after that time, all claims are extinguished. Sealey, 309 Or at 392. Unlike a statute of limitations, the eight-year ultimate repose period prescribed by that statute begins to run on the date on which a product first is purchased for use or consumption, not on the date on which a purchaser knows or should have known of an injury caused by the product. The eight-year statute of ultimate repose runs whether or not a public official or any other plaintiff fails to assert a claim in a timely manner. The public policy for exempting governments from statutes of limitations therefore does not apply to statutes of ultimate repose. That is so, because the expiration of ultimate repose periods extinguishes all claims irrespective of whether the injured plaintiff was negligent in failing to assert claims in a timely manner.
In sum, because ORS 30.905(1) is not premised on whether a plaintiff has filed a claim in a timely manner and because the expiration of the ultimate repose period extinguishes a claim under ORS 30.900 et seq., the common-law exemption that underpins ORS 12.250 does not apply to ORS 30.905(1). We conclude that the common-law variation of ORS 12.250 does not apply to ORS 30.905(1).
We answer Additional Question No. 3 “NO.”
CONCLUSION
For the reasons explained above, we conclude that an irrigation district is a public corporation under ORS 12.250. The exemption in ORS 12.250 does not apply to ORS 30.905(1). The common-law variation of ORS 12.250 does not apply to ORS 30.905(1).
Certified questions and additional question answered.