Meaning the time between the alleged commission of a crime and the filing of charges. If a person's already been charged, convicted, and sentenced, then of course the time has to be served no matter how long it's been.

I remember a case where a man was convicted and sentenced to several years in prison for a robbery, he escaped and moved across the country, started a new life, and lived peacefully (and lawfully) for over 40 years. But the law finally caught up with the old man...and yes he would have been required to return and finish his sentence (but he was granted a pardon in view of the life he had lived)