As a general rule, the Crown is not required to prove beyond a reasonable doubt that the alleged offence occurred within the timeframe set down in the indictment. [FN1]

See R. v. B. (G.)(1990), 56 C.C.C. (3d) 200, at 215-16 (SCC);

Criminal Code, s. 601(4.1): “A variance between the indictment or a count therein and the evidence taken is not material with respect to (a) the time when the offence is alleged to have been committed…”

Typically, when the evidence at trial divulges that the alleged offence occurred at a time outside the timeframe alleged in the indictment/information, the Crown will bring an application under section 601(2) of the Criminal Code to have the indictment amended. 
However, given that the Crown is not required to prove as part of its case that the offence date corresponded with the offence date alleged in the indictment, amendment during trial is not necessarily required in order to secure a conviction. [FN2]

R. v. S.M. 2017 ONCA 878: where the trial judge erred in law in requiring the Crown to prove beyond a reasonable doubt that the offences occurred within the timeframe alleged in the indictment. 

R. v. Smiley (R.R.) (1995), 1995 CanLII 960 (ON CA), 80 O.A.C. 238 [endorsement]: which considered the meaning of the phrase “on or about [date]”, standard wording in an information/indictment.  If the evidence shows an offence to have been committed within some period that has a reasonable proximity to the date alleged in the indictment, the Court may proceed without a formal amendment of the information.

The ultimate question is what effect does the amendment have on the accused? Prejudice to the accused remains the litmus test against which all proposed amendments are judged.

R. v. Irwin, 1998 CanLII 2957 (ON CA).
Where prejudice is established, fairness to the accused may demand that the Crown prove that the offence occurred within the timeframe alleged in the indictment/information. 
Alternatively, if in the opinion of the court the misleading or prejudice may be removed by an adjournment, the court may adjourn the matter under section 601(5) of the Criminal Code.

In some circumstances a court can use its implicit authority to manage and control the proceedings over which it presides to mitigate or eliminate prejudice to the accused, for instance, by permitting the defence to recall witnesses.
[FN1] For some offences, however, the age of the complainant is an essential element of the offence (eg. invitation to sexual touching).  Therefore, in some prosecutions the Crown must establish beyond a reasonable doubt the timeframe in which the offence was committed in order to establish that the complainant was of a particular age.

[FN2] Though amending the indictment before the defence calls evidence may reduce the prejudice to the accused.

Note: Amendment of the indictment is not restricted to trial: under section 601(3), a court may at any stage of the proceedings amend the indictment or a count therein as may be necessary.
Criminal Code

Amendment where variance

601(2) Subject to this section, a court may, on the trial of an indictment, amend the indictment or a count therein or a particular that is furnished under section 587, to make the indictment, count or particular conform to the evidence, where there is a variance between the evidence and

(a) a count in the indictment as preferred; or

(b) a count in the indictment

(i) as amended, or

(ii) as it would have been if it had been amended in conformity with any particular that has been furnished pursuant to section 587.

Matters to be considered by the court

(4) The court shall, in considering whether or not an amendment should be made to the indictment or a count in it, consider

(a) the matters disclosed by the evidence taken on the preliminary inquiry;

(b) the evidence taken on the trial, if any;

(c) the circumstances of the case;

(d) whether the accused has been misled or prejudiced in his defence by any variance, error or omission mentioned in subsection (2) or (3); and

(e) whether, having regard to the merits of the case, the proposed amendment can be made without injustice being done.