In any case in which a person charged with a crime intends (i) to put in issue his sanity at the time of the crime charged and (ii) to present testimony of an expert to support his claim on this issue at his trial, he, or his counsel, shall give notice in writing to the attorney for the Commonwealth, at least 60 days prior to his trial, of his intention to present such evidence. However, if the period between indictment and trial is less than 120 days, the person or his counsel shall give such notice no later than 60 days following indictment. In the event that such notice is not given, and the person proffers such evidence at his trial as a defense, then the court may in its discretion, either allow the Commonwealth a continuance or, under appropriate circumstances, bar the defendant from presenting such evidence. The period of any such continuance shall not be counted for speedy trial purposes under § 19.2-243.