Defendant Jamie Ross appeals his conviction of sexual assault upon a minor in violation of 13 V.S.A. § 3252(3). He claims five errors: (1) a deficient information charging the time of offense, (2) denial of his request to have a defense expert examine the victim, (3) improper bolstering of the victim’s credibility by the State, (4) infringement of his right to confront the victim on cross-examination, and (5) an unfair demand by the State at sentencing. We affirm.
I.
The Information
In early 1984, while in foster care, the victim, age eight, told *465her foster mother that both her stepfather and her uncle, defendant-appellant here, had sexually abused her. At trial, the victim described acts of sexual intercourse with her by defendant in her bedroom. The sexual abuse was corroborated by her younger brother, who said he witnessed some of it. The charging document alleged that the sexual assault occurred during the “summer of 1983.” The court denied defendant’s motion to dismiss the information on the ground that the time of the offense was too indefinite to adequately prepare a defense.
Our law recognizes that sex-related crimes — especially those involving child victims — are often hard to identify as happening on a particular date. Consequently, time is not an essential element to be charged in the information. See State v. Williams, 137 Vt. 360, 362, 406 A.2d 375, 376 (1979), cert. denied, 444 U.S. 1048 (1980). So long as it is shown that the crime occurred at a time not barred by the statute of limitations and the defendant is given fair opportunity to prepare a defense in light of the circumstances of the case, the defendant is not entitled to a dismissal. See State v. Daniels, 129 Vt. 143, 144, 274 A.2d 480, 480 (1971).
The due process right to notice of the time of the offense varies according to the age and circumstances of the victim and how the sexual abuse was allegedly carried out. Here, the victim was seven, and she did not report the abuse for about eight months after it happened. It is not surprising or unreasonable that the State could not determine the time of the offense any more precisely than it did.
Defendant argues that the vagueness of the notice as to time of offense prevented him from establishing an alibi and could subject him to double jeopardy. The abuse alleged here was behavior repeated over a period of time. An alibi in a case like this requires a showing of lack of opportunity over the whole time the alleged abuse took place. Defendant claimed he was never alone with the victim because his wife was always there. His opportunity to establish an alibi was not unreasonably thwarted.
*466As to the double jeopardy claim, defendant’s argument is premature. “The proper time and method to challenge a subsequent charge on the grounds of former jeopardy is at the time such a charge is instituted —” State v. Nash, 144 Vt. 427, 435, 479 A.2d 757, 761 (1984).
II.
Reciprocal Discovery
Defendant asked that a defense expert be permitted to examine the victim to evaluate whether she was suffering from post-traumatic stress disorder (PTSD) syndrome as a result of sexual abuse or other causes. The court denied the request. The State’s expert, who had examined the victim, testified over defendant’s objection. We have previously approved the use of expert testimony on PTSD in a limited way to aid the jury in the understanding of sexual abuse. State v. Catsam, 148 Vt. 366, 369-70, 534 A.2d 184, 187 (1987).
Defendant maintains, however, that when the defense does not have the opportunity to counter the State’s expert with its own, the State should not be permitted to present its expert to the jury. We foresee that there will be cases where it is appropriate either to allow examination of a victim by the defense expert or to prohibit the State from providing its expert testimony when the former procedure is inappropriate. But this is not such a case. Such decisions are left to the sound discretion of the trial court, which was not, on this record, abused. Cf. State v. Percy, 149 Vt. 623, 636-38, 548 A.2d 408, 415-17 (1988) (due process right to reciprocal discovery not violated where court permitted State expert to interview defendant but defendant’s expert was not permitted to interview victim).
The purpose of defendant’s proposed expert presentation of the victim’s mental state might have been either to persuade the jury that she did not suffer from PTSD or, if she did, to show that it was caused by someone other than the defendant. As to the first point, the defense argued to the jury that the victim had been sexually assaulted repeatedly over at least a two-year period by her stepfather, who had been convicted of the crime. The victim, according to the defendant, showed many of the symptoms associated with PTSD. The *467defense portrayed a little girl who had been so victimized sexually by her stepfather that “she may never be made whole.” As to the latter point, the State’s expert admitted that she could not tell from the PTSD symptoms alone who had abused the victim, only that she had been sexually abused. There was, therefore, no need in this case for defendant’s expert to examine the victim, for there was no dispute over the sole legitimate conclusion reached by the State’s expert— that the victim suffered from PTSD syndrome caused by sexual abuse.
III.
Victim’s Credibility
Defendant contends that the trial court erred in permitting the State to introduce expert testimony on the credibility of the child victim.
Dr. Pamela Langelier, a psychologist, was called by the State as an expert witness to explain to the jury the results of her evaluation of the victim. She related the psychological symptomatology of a sexually abused child of the victim’s age. She was asked, “Is it your experience in dealing with children who are sexually abused that these symptoms ... can be easily faked?” Dr. Langelier stated that they could not, “not at a young age,” and added, “Common sense tells you that little children don’t make up stories or fake sexual knowledge. They have no interest in doing it. They gain themselves nothing.”
Dr. Langelier told the jury that the victim was “referred to me to determine whether or not she had been sexually abused ... to what extent, by whom, and indeed if what she was saying is what happened.” The witness then proceeded to detail the victim’s sexual activity with defendant as told to her by the victim and corroborated by a foster parent. Dr. Langelier concluded that the victim had been sexually abused by defendant and others:
So, I think I saw enough to believe that I saw symptomatology indicating sexual abuse had gone on for anywhere from four to five years, but the frequency of which it’s hard to gain [sic] because of the environment in which she was raised in the first place. But based on her own records, it seems that the frequency with Uncle Jamie was *468not as frequent as it was with other parties, but that it was defined with Uncle Jamie with specific memory because it was unusual form of stimulation for a child that age, oral sex. So, she remembered it clearly and it looks like it was twice if not more.
Defendant made no objection to the above testimony. Insofar as it amounted to an opinion that the victim had suffered sexual abuse, such an opinion would not be inadmissible. See Cadel v. Sherburne Corp., 139 Vt. 134, 137, 425 A.2d 546, 547-48 (1980) (opinion evidence otherwise admissible is not objectionable because it embraces an ultimate issue to be decided by the trier of fact); State v. Norton, 134 Vt. 100, 104, 353 A.2d 324, 326 (1976) (same); V.R.E. 704 (same).
It is a closer call, however, as to whether the testimony, taken as a whole, amounted to a conclusion on the part of the witness about the guilt or innocence of defendant. In effect, the testimony was a summation of the witness’s own observations and records, including what the victim herself had said about what had happened to her. To the extent that the testimony amounted to a comment on the credibility of the victim, it would ordinarily be inadmissible. State v. Catsam, 148 Vt. at 370-71, 534 A.2d at 187-88. Inasmuch as defendant did not object, however, he failed to preserve the issue for appeal, and we are bound to reverse only if we find that the trial court’s failure to exclude the testimony sua sponte constituted plain error.1 State v. Recor, 150 Vt. 40, 46, 549 A.2d 1382, 1387 (1988); State v. DeJoinville, 145 Vt. 603, 605, 496 A.2d 173, 174-75 (1985). Plain error will be found “only in a rare and extraordinary case where the error is an obvious one,” State v. Ramsay, 146 Vt. 70, 75, 499 A.2d 15, 18 (1985), and only if the error affects substantial rights of the defendant. Recor, 150 Vt. at 46, 549 A.2d at 1387; V.R.Cr.P. 52(b).
We have previously encountered allegations of plain error under similar circumstances in DeJoinville and Reeor. In DeJoinville, the expert witness testified that children would *469not lie about being sexually abused, and the prosecutor twice repeated the substance of this testimony to the jury in closing argument. DeJoinville, 145 Vt. at 604, 496 A.2d at 174. The defendant made no objection at trial either to the testimony or to the prosecutor’s remarks, but oh appeal, argued that the court had committed plain error by not excluding the various statements sua sponte. We held there was no plain error, noting that plain error will be found “‘only in rare and extraordinary cases’” and that “‘the error must be obvious and strike at the very heart of defendant’s constitutional rights or result in a miscarriage of justice.’” Id. at 605, 496 A.2d at 175 (quoting State v. Turner, 145 Vt. 399, 403, 491 A.2d 338, 340 (1985)).
In Reeor, the expert witness, when asked what it meant when a child gave consistent stories over several interviews, testified that it gave the expert a sense that “what they are saying happened, happened.” Recor, 150 Vt. at 45, 549 A.2d at 1386. That testimony, which was held not to amount to plain error, id. at 46, 549 A.2d at 1387, is of exactly the same tenor as the testimony in the instant case. In short, there is little to distinguish this case from either DeJoinville or Reeor. If anything, DeJoinville is more egregious than the instant case because of the repeated emphasis therein placed on the expert’s testimony by the prosecutor in closing argument.
The issue concerning the scope of Dr. Langelier’s testimony was raised by the court at the outset of trial. Prior to opening statements, the court inquired about the State’s expert on child sexual abuse, as follows, “[S]he’s not going to testify on the ultimate issue of credibility?” The prosecutor answered, “No, she’s not----The State in no way is going to solicit testimony on the credibility issue. We understand how bad that is.” It is clear, then, that both the court and counsel had the issue very much in mind. If there were an obvious violation, it should have been noticed by counsel; yet, when the testimony was given, counsel made no objection.
As we have stated previously, ‘“[t]he duty to exclude objectionable data lies squarely upon the shoulders of defense counsel.’” State v. Kasper, 137 Vt. 184, 190, 404 A.2d 85, 89 (1979) (quoting United States v. Castenada, 555 F.2d 605, 610 (7th Cir. 1977)). “‘If the rule were otherwise, counsel might *470at times be tempted to remain silent about some fault on the part of the trial court ... and so, without giving it a chance to correct the situation, arm themselves with ground for reversal if the verdict should go against them.’” Id. at 190-91, 404 A.2d at 89 (quoting State v. Hood, 123 Vt. 273, 277-78, 187 A.2d 499, 502 (1963)).
We note a further distinction between the instant case and Catsam, relied upon by defendant. In Catsam, the Court noted: “While admission of this type of expert testimony might be harmless in a case in which there was ample extrinsic evidence of guilt, that is not the case here.” Catsam, 148 Vt. at 372, 534 A.2d at 188. The only direct evidence of the offense in Catsam was the testimony of the child-victim herself. In the case before us, there was extrinsic evidence of guilt in the form of an eyewitness to the offense itself: the brother of the victim. It is difficult to imagine stronger, more direct, extrinsic evidence in a criminal case than the testimony of a person who actually witnessed the crime in progress.
On the basis of the record before us and our own prior precedents, we conclude that the testimony of Dr. Langelier did not amount to plain error.
IV.
Right to Confront Witness
The defense, prior to trial, filed a notice that it intended to introduce evidence that the victim had been sexually assaulted by her stepfather and others. The State moved in limine to exclude evidence of the victim’s prior sexual conduct under the Rape Shield Law, 13 V.S.A. § 3255. During argument before jury selection, defense counsel indicated in somewhat vague terms that several named relatives and other unnamed people, in addition to her stepfather and the defendant, had been accused by the victim of sexually assaulting her, and that she later denied some of these allegations. The court, in a preliminary ruling, indicated an inclination to exclude such evidence, but left the question open to be raised again during the presentation of evidence; defendant did not again raise the issue, however.
*471During the course of the trial, it became apparent that defendant was not the only one alleged to have sexually mistreated the victim. While cross-examining the victim, defense counsel elicited from her that her stepfather had abused her sexually, “[a] lot of times.” When defense counsel inquired about a camping trip with another uncle, the State’s objection was sustained. No offer of proof was made. The defense’s point became apparent when Dr. Langelier testified that another uncle was mentioned by the victim as an abuser. Thus, insofar as demonstrating another possible source of the PTSD syndrome, this issue was amply brought to the jury’s attention through the testimony of both Dr. Langelier and the victim. See State v. Catsam, 148 Vt. at 379, 534 A.2d at 192-93. Although defendant argues that it was error to prevent him from cross-examining the victim about other alleged abusers, we fail to fathom what more impact such an inquiry could have had in defending the charge, absent an offer of proof detailing the basis for the relevance of the inquiry. See V.R.E. 103(a)(2) (error may not be predicated upon a ruling excluding evidence unless “the substance of the evidence was made known to the court by offer”).
Defendant further contends that he should have been permitted to test the victim’s credibility by inquiring about prior false allegations of sexual abuse. The Rape Shield Statute, 13 V.S.A. § 3255, makes evidence of the prior sexual conduct of a complaining witness an improper subject for cross-examination, with certain exceptions, only one of which is pertinent to this discussion. Where the evidence “bears on the credibility of the complaining witness or it is material to a fact at issue and its probative value outweighs its private character, the court may admit ... [e]vidence of specific instances of the complaining witness’ past false allegations of [sexual assault].” § 3255(a)(3)(C). Thus, before evidence of prior sexual conduct may be admitted into evidence, the statute requires that there be specific evidence of past false allegations. Defense counsel conceded to the trial court, however, that he did not know whether the allegations about which he sought to inquire were false or not. On the state of the offer *472as made by counsel, therefore, the court was justified in excluding the evidence.2
V.
Sentencing
Defendant’s final claim of error requires little discussion. At sentencing, the State recommended a sentence of eight to fifteen years. Defendant received what the State asked for. Prior to trial, the prosecution had agreed with the defendant, in exchange for a plea of guilty, to a sentence of two to eight years. The plea agreement was abandoned after the court expressed some doubts about defendant’s competency to enter into it. Defendant now claims that the prosecutor’s recommendation, which was not objected to at sentencing, constitutionally impinged upon his right to a jury trial by punishing him for the exercise of that right. The State was under no obligation, however, to recommend a particular sentence following trial on the merits, because the quid pro quo, defendant’s plea of guilty, had been withdrawn from the bargain. See Blackmon v. Wainwright, 608 F.2d 183, 184 (5th Cir. 1979), reh’g denied, 611 F.2d 882, cert. denied, 449 U.S. 852 (1980) (if defendant turns down plea bargain offered by prosecutor, he may not then complain that he received a heavier sentence after trial, absent evidence of vindictiveness or punitive action); Mitchell v. State, 521 So. 2d 185, 187 (Fla. Dist. Ct. App. 1988) (when accused voluntarily chooses to reject or withdraw from a plea bargain, he retains no right to the rejected sentence). There is no evidence of vindictiveness on the part of the court or prosecutor, nor is there any reason to believe that defendant’s sentence was the product of *473anything other than an appropriate assessment of the circumstances following full disclosure of the facts at trial and in the presentence report. See Miller v. Dugger, 858 F.2d 1536, 1538 (11th Cir. 1988).
Affirmed.
The same expert witness testified at the trial of the victim’s stepfather. Her opinion on the credibility of the victim, which was admitted over objection of counsel, resulted in a reversal and remand for a new trial. State v. Ross, No. 86-017 (Vt. Mar. 8, 1988) (unpublished opinion).
This approach is consistent with that followed elsewhere. See Little v. State, 413 N.E.2d 639, 643 (Ind. App. 1980) (evidence of alleged rape victim’s false accusations of similar sexual misconduct is admissible on issue of victim’s credibility, but allegations must be demonstrably false); Commonwealth v. Bohannon, 376 Mass. 90, 95, 378 N.E.2d 987, 991 (1978) (error for trial court to exclude proffered evidence of other false accusations of rape where defendant made offer of proof that indicated he had factual basis for concluding the prior allegations of rape were untrue); State v. Padilla, 110 Wis. 2d 414, 427, 329 N.W.2d 263, 270 (1982) (not error for court to exclude cross-examination about prior allegations of sexual intercourse with another man where there was no proof the prior allegations were untruthful).