In the imposition of sentence for violation of the penal laws, it shall be the duty of the judge to specify that the term of service under the sentence shall be computed from the date of sentence if the defendant is confined in jail or otherwise incarcerated and has no appeal or motion for new trial pending. In cases which are appealed to the Georgia Court of Appeals or the Georgia Supreme Court for reversal of the conviction, the sentence shall be computed from the date the remittitur of the appellate court is made the judgment of the court in which the conviction is had, provided the defendant is not at liberty under bond but is incarcerated or in custody of the sheriff of the county where convicted. If a defendant has been convicted and sentenced but, because of his failure or inability to post bond or bail for any reason, he has been incarcerated pending the prosecution of an appeal to any court, the time of the original imposition of his sentence until the time when the remittitur of the appellate court is made the judgment of the court in which the conviction is had shall be counted as time spent under sentence for all purposes.