Petitioner filed a petition for post-conviction relief. It was dismissed on the ground that his claim was barred by the Statute of Limitations. He appeals. The issue is whether the 1989 amendment to the Post-Conviction Hearing Act (“the Act”),1 adding a limitation period, applies to petitioner, whose conviction was entered before the amendment’s effective date. We hold that the amendment applies, and we affirm.
Petitioner was convicted of burglary in the second degree. ORS 164.215. His conviction became final on May 23, 1989. At the time, ORS 138.510(2)providedthatapetitionfor post-conviction relief “may be filed without limit in time.” In August, 1989, the legislature amended that section to provide:
‘ ‘A petition pursuant to [the Act] must be filed within 120 days of the following, unless the court on hearing a subsequent petition finds grounds for relief asserted which could not reasonably have been raised in the original or amended petition:
“(a) If no appeal is taken, the date the judgment or order on the conviction was entered in the register. ” Or Laws 1989, ch 1053, § 18.
The amendment became effective when the governor approved SB 284 on August 5,1989. Or Laws 1989, ch 1053, §§ 19, 22. One hundred and sixty-five days later, on January 17,1990, petitioner filed for post-conviction relief.2 The post-conviction court dismissed the petition as untimely.
Petitioner contends that the legislature did not expressly declare whether the 1989 amendment to the Act should be applied to convictions that became final before August 5, 1989. He asserts that the legislature’s silence creates a presumption that the amendment was not intended to apply to convictions entered before its effective date. Defendant contends that the new limitation period applies to *284all post-conviction proceedings, regardless of when the conviction was entered in the register.
A state may limit the time within which a prisoner may seek post-conviction relief. See, e.g., United States v. Randolph, 262 F2d 10 (7th Cir 1958), cert den 359 US 1004 (1959). Therefore, the sole issue here is whether the legislature intended the limitation that it established in 1989 to apply to convictions entered before its effective date.3
Legislative silence has generally led Oregon appellate courts to construe statutory amendments to only apply prospectively. See State v. Burke, 109 Or App 7, 10, 818 P2d 511 (1991). This is true,
‘ ‘without respect to whether the change might be ‘procedural or remedial’ or ‘substantive’ in a strictly technical sense.” Joseph v. Lowery, 261 Or 545, 549, 495 P2d 273 (1972).
However, with the exception of ex post facto laws, the legislature may enact legislation intended to apply retroactively. See Whipple v. Howser, 291 Or 475, 480, 632 P2d 782 (1981).
In determining whether the 1989 amendment applies to convictions entered before its effective date, our duty is to ascertain the legislature’s intent. ORS 174.020; State v. Galligan, 312 Or 35, 39, 816 P2d 601 (1991). Our first step is to examine the language of the statute itself. ORS 174.010; State ex rel Juv. Dept. v. Ashley, 312 Or 169, 174, 818 P2d 1270 (1991).
ORS 138.510(1) provides:
“Except as otherwise provided in ORS 138.540, any person convicted of a crime under the laws of this state may file a petition for post-conviction relief pursuant to [the Act].”
ORS 138.540(1) provides, in relevant part:
“Except as otherwise provided in [the Act], a petition pursuant to [the Act] shall be the exclusive means, after judgment rendered upon a conviction for a crime, for challenging the lawfulness of such judgment or the proceedings upon which it is based.”
*285The plain meaning of the words in ORS 138.540(1) is that a petition pursuant to the Act is the exclusive means for challenging the lawfulness of a conviction, unless the Act specifies otherwise. The Act does not distinguish between convictions that became final before August 5,1989, and convictions that became final after that date. Consequently, judgments entered before August 5, 1989, are not “otherwise provided [for] in [the Act].” ORS 138.540(1) requires that challenges to those convictions, like any other, must be made “pursuant to the Act.”
The Act now provides a single 120-day time frame for filing a post-conviction petition. If the petition must be filed pursuant to the Act, then this petition, like any other, must have been filed within that time frame, unless the grounds for relief “could not reasonably have been raised” earlier. ORS 138.510(2).
Defendant contends that the legislature expressly provided for a collateral attack on convictions entered before August 5,1989, in ORS 138.510(2). In this regard, defendant says:
“What the legislature did do, however, was to include a general provision that allows an untimely petition (i.e., one not filed within 120 days after the conviction became final) if the claims asserted therein ‘could not reasonably have been raised in the original or amended petition’ (e.g., the petitioner has some legitimate excuse for not filing the petition in a timely manner). That provision provides a mechanism for a petitioner whose post-conviction petition otherwise would be barred by strict application of the new 120-day rule to avoid the bar.”
The Act now provides that an untimely petition may be filed if
“the court on hearing a subsequent petition finds grounds for relief asserted which could not reasonably have been raised in the original or amended petition * * ORS 138.510(2).
Although the legislature may have intended to permit the late filing of a petition on the basis of some kind of good cause standard, the language used to convey that message is less than straightforward. The language in ORS 138.510(2), read literally, appears to dictate that an untimely petition, whatever its merits, can be considered only if an earlier petition *286was timely filed. We need not construe the meaning of that language, because petitioner did not allege, and does not contend, that his petition fits within the purported exception to the 120-day limitation period.
Defendant contends that the new limitation may be applied to all persons convicted before August 5,1989, so long as they are given a reasonable amount of time after that date to file a post-conviction petition. Defendant asserts that the new 120-day limit also measures a reasonable amount of time to challenge convictions entered before that date.4
Generally, determination of what is a reasonable amount of time is within the legislature’s purview. Texaco, Inc. v. Short, 454 US 516, 527 n 21, 102 S Ct 781, 70 L Ed 2d 738 (1982) (citing Wilson v. Iseminger, 185 US 55, 22 S Ct 573, 46 L Ed 804 (1902)). However, the Oregon Supreme Court declared some time ago:
“If the legislature should pass an act, barring a past action, without any allowance of time for the institution of a suit in future, such an act would be so unreasonable as to amount [to] a denial of [a] right, and call for the interposition of the court.
“We conclude, then, that * * * the court is bound, by the fundamental law, to give a party reasonable time in which to escape the effect of such remedy.” McLaughlin v. Hoover, 1 Or 31, 34 (1853). (Emphasis supplied.)
Oregon courts have not heretofore declared how to measure a “reasonable time” when the legislature has neglected to do so explicitly. We find North Carolina’s approach instructive. When the legislature shortens a limitation period,
“a reasonable time [to commence a suit after the effective date] shall be the balance of the time [remaining under the old law], provided it shall never exceed the time allowed by the new statute.” Spaulding v. R.J. Reynolds Tobacco Co., 93 NC App 770, 379 SE2d 49, 51 (1989), aff’d 326 NC 44, 387 SE2d 168 (1990).
*287Oregon case law is consistent with that formula. Each time that appellate courts have gauged a “reasonable time” for bringing actions after the legislature shortened a limitation period, the “reasonable time” either equalled, or was less than, the new limitation period. See Evans v. Finley, 166 Or 227, 111 P2d 833 (1941); McLaughlin v. Hoover, supra; Wolf v. Goin, 26 Or App 23, 552 P2d 258, rev den 276 Or 133 (1976).
The 120-day period, from August 5 to December 3, 1989, provided a reasonable window of opportunity for petitioner to seek relief. He failed to take advantage of that opportunity. We are persuaded that the statute evinces the legislature’s intent to apply the limitation period uniformly to all convictions, regardless of when they were entered in the register. Therefore, we hold that the post-conviction court correctly dismissed the petition.
Affirmed.
Post-Conviction Hearing Act, ORS 138.510 —ORS 138.680.
The petition alleges only that petitioner was denied effective assistance of counsel, because his trial attorney did not object to imposing restitution. Petitioner does not allege any “grounds for relief which could not reasonably have been raised,” ORS 138.510(1), within 120 days after either entry of judgment or enactment of the limitation period.
Petitioner raises no constitutional challenges to the new limitation period.
The Department of Justice represents in this case that it has not moved to dismiss any petition filed by December 3, 1989, which was the end of the 120-day period commencing on the amendment’s effective date.