OS X 10.10 and later only. This script will reset the Start and (optionally) the Stop Time of the selected tracks--or all the tracks of the selected playlist--to "0:00" and (optionally) their entire duration, respectively.
You might also be interested in Batch Trim Start or Stop Time (Seconds).
Choose to reset the tracks Start Time to 0:00 and, optionally, the Stop Time to the full duration of the track
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