delivered the opinion of the Court.
Petitioner Robert Toussie was convicted, after a jury-trial, of failing to register for the draft. His conviction was affirmed by the Court of Appeals, 410 F. 2d 1156 *113(C. A. 2d Cir.), and we granted certiorari, 396 U. S. 875 (1969). For the reasons hereafter set forth we conclude that this prosecution was barred by the statute of limitations and therefore reverse the conviction.
Section 3 of the Universal Military Training and Service Act, 65 Stat. 76, provides that:
“Except as otherwise provided in this title, it shall be the duty of every male citizen . . . who, on the day or days fixed for the first or any subsequent registration, is between the ages of eighteen and twenty-six, to present himself for and submit to registration at such time or times and place or places, and in such manner, as shall be determined by proclamation of the President and by rules and regulations prescribed hereunder.” 1
The applicable presidential proclamation provides that “[p]ersons who were born on or after September 19, 1930, shall be registered on the day they attain the eighteenth anniversary of the day of their birth, or within five days thereafter.”2 Since Toussie, an American citizen, was born on June 23, 1941, he was required to register sometime between June 23 and June 28, 1959. He did not do so during that period or at any time thereafter. On May 3, 1967, he was indicted for failing to register and that indictment led to the conviction under review.
*114Before trial Toussie moved to dismiss the indictment, arguing that prosecution was barred by the statute of limitations which provides that “[ejxcept as otherwise expressly provided by law, no person shall be prosecuted, tried, or punished for any offense, not capital, unless the indictment is found . . . within five years next after such offense shall have been committed.” 18 U. S. C. § 3282. Since there is no express provision to the contrary in the Draft Act, Toussie argued that his crime was complete in 1959, and it could not be the subject of a prosecution based on an indictment returned in 1967— eight years thereafter. The Government agreed that the crime was complete in 1959, but argued that it continued to be committed each day that Toussie did not register. The District Court held that the Act imposes a continuing duty to register which lasts until age 26 and that prosecution for failing to perform that duty before the man becomes 26 is timely if the indictment is returned before the defendant becomes 31 years old— in this case any time prior to June 23, 1972. 280 F. Supp. 473, 474 (D. C. E. D. N. Y. 1967). The Court of Appeals agreed. 410 F. 2d, at 1157-1158. If the offense is a continuing one the prosecution was timely, but, if not, the District Court erred in not dismissing the indictment.
In deciding when the statute of limitations begins to run in a given case several considerations guide our decision. The purpose of a statute of limitations is to limit exposure to criminal prosecution to a certain fixed period of time following the occurrence of those acts the legislature has decided to punish by criminal sanctions. Such a limitation is designed to protect individuals from having to defend themselves against charges when the basic facts may have become obscured by the passage of time and to minimize the danger of official punishment *115because of acts in the far-distant past. Such a time limit may also have the salutary effect of encouraging law enforcement officials promptly to investigate suspected criminal activity. For these reasons and others, we have stated before “the principle that criminal limitations statutes are 'to be liberally interpreted in favor of repose,' United States v. Scharton, 285 U. S. 518, 522 (1932).” United States v. Habig, 390 U. S. 222, 227 (1968). We have also said that “[s]tatutes of limitations normally begin to run when the crime is complete.” Pendergast v. United States, 317 U. S. 412, 418 (1943); see United States v. Irvine, 98 U. S. 450, 452 (1879). And Congress has declared a policy that the statute of limitations should not be extended “[ejxcept as otherwise expressly provided by law.” 18 U. S. C. § 3282. These principles indicate that the doctrine of continuing offenses should be applied in only limited circumstances since, as the Court of Appeals correctly observed in this case, “[tjhe tension between the purpose of a statute of limitations and the continuing offense doctrine is apparent; the latter, for all practical purposes, extends the statute beyond its stated term.” 410 F. 2d, at 1158. These considerations do not mean that a particular offense should never be construed as a continuing one. They do, however, require that such a result should not be reached unless the explicit language of the substantive criminal statute compels such a conclusion, or the nature of the crime involved is such that Congress must assuredly have intended that it be treated as a continuing one.
The statute in this case provides that all young men, with certain exceptions, between the ages of 18 and 26 shall register “at such time or times and place or places” as the President may prescribe. The Government refers to a regulation promulgated under the Act which pro*116vides that “[t]he duty of every person subject to registration . . . shall continue at all times, and if for any reason any such person is not registered on the day or one of the days fixed for his registration, he shall immediately present himself for and submit to registration . . . 32 CFR § 1611.7 (c). It is urged that this regulation only makes explicit what Congress implicitly said in the Act itself, that is that registration is a duty that continues until age 26 and failure to register before then is a criminal offense that can be punished as late as five years after the 26th birthday.
The statute admittedly might be construed as the Government urges, but in light of the history of the draft laws and the principle that continuing offenses are not to be too readily found, we do not feel this particular Act incorporates such a doctrine. The draft law of 1917 provided in § 5 that certain persons were subject to registration and that “upon proclamation by the President . . . stating the time and place of such registration it shall be the duty of all [such] persons ... to present themselves for and submit to registration.” 40 Stat. 80. Pursuant to that authority the President proclaimed June 5, 1917, as the first registration day,3 and on that day approximately 10,000,000 young men were registered.4 There were no more general draft registrations until August 24, 1918, when the President required all those men who had become subject to registration since June 5, 1917, to come in and register.5 Later that year Congress amended the statute, expanded the age group subject to registration,6 and provided that “upon *117proclamation by the President . . , stating the time or times and place or places of . . . registration, it shall be the duty of all persons of the designated ages ... to present themselves for and submit to registration . . . .” 40 Stat. 955-956. Although this provision seemingly would have authorized registrations on different days, the President again issued a proclamation designating a single day, September 12, 1918, as registration day for all those so subject.7 That registration was the last under the World War I draft. It is thus clear that throughout the administration of the first draft law, registration was thought of as a single, instantaneous act to be performed at a given time, and failure to register at that time was a completed criminal offense.
As events developed prior to what became World War II, Congress again decided to draft young men for service in the Armed Forces. In the Selective Training and Service Act of 1940 it was provided that men subject to registration were to register “at such time or times and place or places, and in such manner and in such age group or groups, as shall be determined by rules and regulations prescribed hereunder.” 54 Stat. 885. While this language would again have authorized registration on different days for different men, the first proclamation under the new Act set a uniform date, October 16, 1940, for the registration of all men.8 It was not until two years later that the President first issued a proclamation setting forth different dates for the registration of different groups of men, and in that same proclamation the President established the basic registration procedure of the present system, that all young men shall register on their 18th birthday.9
*118After the 1940 Act expired on March 31, 1947, Congress again decided to register men for the draft and declared that men between the ages of 18 and 26 would be subject to registration. Selective Service Act of 1948, 62 Stat. 604. Since the authority to register under the 1940 Act had expired, it was necessary to provide for the initial registration of the entire group of men between 18 and 26. In language identical to that found in the statute involved in this case,10 Congress again left the administrative details to the President and authorized registration “at such time or times and place or places” as he might designate. We do not think the imposition of the duty to register on men between 18 and 26 and the provision for registration at different times was intended to indicate that the statute of limitations did not begin to run when the crime was first complete. Since at the time of the initial registration under the 1948 Act there were men of various ages who had to be registered, the Act was phrased generally in terms of a duty imposed on the entire group. Under this authority the President in fact required registration of all men between 18 and 26 during the month of September 1948, Persons of different ages were required to register on different days, and all those born after September 19, 1930, were required to register “on the day they attain the eighteenth anniversary of the day of their birth, or within five days thereafter.” 11 The registration provisions of that Act have remained in force since 1948, and there has thus been a continual registration of 18-year-olds shortly after their birthday. With the exception of a few men who are not subject to registration when they are 18 but may become *119so later on,12 the effect of these provisions has been to eliminate the necessity for registrations of men older than 18. Viewed in the light of history we do not think the Act intended to treat continued failure to register as a renewal of the original crime or the repeated commission of new offenses, but rather perpetuated the conception of the first registration that a man must register at a particular time and his failure to do so at that time is a single offense. That time will not be the same day for all as it was in 1917, and from the Selective Service System’s viewpoint the process of registration is a “continuing” one. But from the registrant’s viewpoint the obligation arises at a specific time. In Toussie’s case it arose when he turned 18. He was allowed a five-day period in which to fulfill the duty, but when he did not do so he then and there committed the crime of failing to register.
The Government points out that the “continuing duty” regulation has been in existence since before the passage of the 1948 Act,13 and that most lower federal courts have held that failing to register is a continuing offense for purposes of applying the statute of limitations.14 It is suggested that since Congress has legislated *120several times in this field, its failure to indicate that the crime should not be treated as a continuing offense supports the Government's argument that it is. Petitioner on the other hand suggests that Congress has on occasion explicitly stated that a certain offense will be deemed a continuing one,15 and its failure to do so in this statute indicates that it did not intend to adopt that theory. Since there is no specific evidence that Congress actually was aware of this limitations question when it acted— whatever weight such evidence might deserve — and since we are reluctant to imply a continuing offense except in limited circumstances, we conclude that any argument based on congressional silence is stronger in favor of not construing this Act as incorporating a continuing-offense theory.
Unlike other instances in which this Court has held that a particular statute describes a continuing offense, there is no language in this Act that clearly contemplates a prolonged course of conduct.16 While it is true that *121the regulation does in explicit terms refer to registration as a continuing duty, we cannot give it the effect of making this criminal offense a continuing one. Since such offenses are not to be implied except in limited circumstances, and since questions of limitations are fundamentally matters of legislative not administrative decision, we think this regulation should not be relied upon effectively to stretch a five-year statute of limitations into a 13-year one, unless the statute itself, apart from the regulation, justifies that conclusion.17
*122There is also nothing inherent in the act of registration itself which makes failure to do so a continuing crime. Failing to register is not like a conspiracy which the Court has held continues as long as the conspirators engage in overt acts in furtherance of their plot. See United States v. Kissel, 218 U. S. 601 (1910), Grunewald v. United States, 353 U. S. 391 (1957). It is in the nature of a conspiracy that each day’s acts bring a renewed threat of the substantive evil Congress sought to prevent. The fact that the first draft registrations clearly were viewed as instantaneous events and not a continuing process indicates that there is nothing inherent in the nature of failing to register that makes it a continuing offense.
We do not mean that the argument in support of implying a continuing offense in this case is insubstantial, but it is at best highly equivocal. Basically we are faced with the task of construing a somewhat ambiguous statute in one of two ways. One way would limit institution of prosecution to a period of five years following the initial violation, while the other could effectively extend the final date for prosecution until as late as 13 years after the crime is first complete. As we have said before:
“when choice has to be made between two readings of what conduct Congress has made a crime, it is appropriate, before we choose the harsher alternative, to require that Congress should have spoken in language that is clear and definite. We should not derive criminal outlawry from some ambiguous implication.” United States v. Universal Corp., 344 U. S. 218, 221-222 (1952).
Not insignificantly those remarks were also made in the context of considering the continuing-offense doctrine. In light of all these considerations we conclude that the *123draft law does not intend to permit criminal prosecution for failing to register as late as 13 years after the initial failure. Consequently the statute of limitations begins to run at the initial failure to register as required by law. Since the facts in this case clearly show that Toussie failed in his legal obligation when he did not register prior to June 28, 1959, the statute began to run at that time and prosecution based on an indictment returned almost eight years later was barred.
It should be emphasized that this conclusion does not mean that the gravity of this offense is in any way diminished. Failure to register is subject to heavy criminal penalties. The only question is whether those penalties must result from a prosecution begun within five years or whether they can be delayed for a longer period. We are not convinced that limiting prosecution to a period of five years following the initial failure to register will significantly impair either the essential function of raising an army or the prosecution of those who fail to register. We do feel that the threat of criminal punishment and the five-year statute of limitations is a sufficient incentive to encourage compliance with the registration requirements. If Congress had felt otherwise it could easily have provided for a longer period of limitations. It has not yet done so.
There is no doubt that the jury found that Toussie willfully failed to register and thereby subject himself to the same possibility of military service that faces other young men who fully comply with their legal obligations. There is some cause to feel that dismissal of the indictment in such a case is an injustice in a society based on full and equal application of the laws. But while Congress has said that failure to register is a crime, it has also made prosecution subject to the statute of limitations. “Every statute of limitations, of course, may permit a rogue to escape,” Pendergast v. United States, *124317 U. S. 412, 418 (1943), but when a court concludes that the statute does bar a given prosecution, it must give effect to the clear expression of congressional will that in such a case “no person shall be prosecuted, tried, or punished.” The judgment of conviction in this case must therefore be
Reversed.
50 U. S. C. App. § 453. This Act was amended by the Military Selective Service Act of 1967, 81 Stat. 100, but those amendments did not change this provision. Failure to perform this duty is punishable by fine, imprisonment, or both. 50 U. S. C. App. § 462 (a) (1964 ed., Supp. IV).
Proclamation No. 2799, July 20, 1948, 62 Stat. 1531. The Proclamation was first issued under the authority of the Selective Service Act of 1948, 62 Stat. 604, but it was continued after the passage of the Universal Military Training and Service Act by Proclamation No. 2942, August 30, 1951, 65 Stat. c35.
Proclamation of May 18, 1917, 40 Stat. 1664.
U. S. Selective Service System, Registration and Selective Service 11 (1946).
Proclamation of August 13, 1918, 40 Stat. 1834.
The first registration was of all men between the ages of 21 and 30. 40 Stat. 80. In 1918 Congress expanded the group to all those between the ages of 18 and 45. 40 Stat. 955.
Proclamation of August 31, 1918, 40 Stat. 1840.
Proclamation No. 2425, September 16, 1940, 54 Stat. 2739.
Proclamation No. 2572, November 17, 1942, 56 Stat. 1982.
See supra, at 113.
See supra, at 113, and Proclamation No. 2799, July 20, 1948, 62 Stat. 1531.
For example, students at certain military colleges are exempted from registration. 50 U. S. C. App. § 456 (a) (1) (1964 ed., Supp. IV). If a student in such an institution withdraws, he would presumably be required to register since the Act specifically states that “[n]o exemption from registration . . . shall continue after the cause therefor ceases to exist.” 50 U. S. C. App. § 456 (k). Thus such a student may not be required to register until some time after his 18th birthday.
The regulation was first promulgated under the 1940 Act on June 4, 1941. Selective Service System Regulations Vol. 2, § IX, 205 (d), 6 Fed. Reg. 2747.
See Fogel v. United States, 162 F. 2d 54 (C. A. 5th Cir.), cert. denied, 332 U. S. 791 (1947); Gara v. United States, 178 F. 2d 38, 40 (C. A. 6th Cir. 1949), aff’d by an equally divided Court, *120340 U. S. 857 (1950); McGregor v. United States, 206 F. 2d 583 (C. A. 4th Cir. 1953); cf. United States v. Guertler, 147 F. 2d 796 (C. A. 2d Cir. 1945). But cf. United States v. Salberg, 287 F. 208 (D. C. N. D. Ohio 1923).
Congress has provided that concealment of a bankrupt’s assets shall “be deemed to be a continuing offense . . . and the period of limitations shall not begin to run until . . . final discharge or denial of discharge.” 18 U. S. C. § 3284.
Cf. United States v. Cores, 356 U. S. 405 (1958), in which the Court held, for venue purposes, that the statute prohibiting alien crewmen from remaining in the United States after their permits expired contemplated that the offense would continue as long as the crewman remained in this country and the statute of limitations did not start to run when he first overstayed his permit. In that case we stated that “[s]ection 252 (c) punishes '[a]ny alien crewman who willfully remains in the United States in excess of the number of days allowed.’ The conduct proscribed is the affirmative act of willfully remaining, and the crucial word 'remains’ permits no connotation other than continuing presence.” Id., at 408. See also Armour *121Packing Co. v. United States, 209 U. S. 56 (1908), in which we held that, for venue purposes, violations of the Elkins Act, 32 Stat. 847, were continuing offenses. In that case the statute specifically provided that “[e]very violation . . . shall be prosecuted in any court of the United States having jurisdiction of crimes within the district in which such violation was committed or through which the transportation may have been conducted . . . .” Id., at 73. Both of these cases dealt with venue and did not involve the statute of limitations question presented in this case.
It is significant that the courts that have concluded that failure to register is a continuing offense have done so by relying explicitly on the regulation. See Fogel v. United States, supra, at 55; McGregor v. United States, supra, at 584; Gara v. United States, supra, at 39; and the opinions below in this case, 280 F. Supp., at 474, 410 F. 2d, at 1157. It is equally significant that the only court that concluded that the offense was not a continuing one did so at a time when there was no “continuing-duty” regulation issued to implement the registration provisions. United States v. Salberg, supra, interpreting the 1917 Draft Act, held that failure to register was not a continuing offense. The first continuing-duty regulation was promulgated in 1941. See n. 13, supra. These decisions support our conclusion that the statute itself, apart from any reliance on the administrative regulation, does not require that it be construed to incorporate a continuing-offense theory. We do not hold, as the dissent seems to imply, post, at 127, that the continuing-duty regulation is unauthorized by the Act. All we hold is that neither the regulation nor the Act itself requires that failure to register be treated as the type of offense that effectively extends the statute of limitations.