(a) A judgment imposing a sentence of imprisonment shall specify whether the sentence is to be served concurrently with or consecutively to any other sentence imposed at the same time or prior thereto. If the sentences are to be served consecutively, the judgment shall specify when each sentence is to begin with reference to the termination of any other sentence.
(b) Unless the judgment specifically postpones the commencement of a sentence pursuant to subsection (a) of this section, such sentence shall commence to run on the date fixed in section 3673 of this title.