(a) Commencement of Probation. - Except as provided in subsection (b), a period of probation commences on the day it is imposed and runs concurrently with any other period of probation, parole, or imprisonment to which the defendant is subject during that period.
(b) Consecutive and Concurrent Sentences. - If a period of probation is being imposed at the same time a period of imprisonment is being imposed or if it is being imposed on a person already subject to an undischarged term of imprisonment, the period of probation may run either concurrently or consecutively with the term of imprisonment, as determined by the court. If not specified, it runs concurrently.