5Where the person convicted on retrial is sentenced to imprisonment or other detention, the sentence shall begin to run from the time when such a sentence passed at the original trial would have begun to run ; but, in computing the term of his sentence or the period for which he may be detained thereunder, as the case may be, there shall be disregarded—
(a)any time before his conviction on retrial which would have been disregarded if the sentence had been passed at the original trial and the original conviction had not been quashed; or
(b)any time during which he was at large after being admitted to bail under section 17 of this Act.