(a) Commencement of Sentence.— A sentence
to a term of imprisonment commences on the date the defendant is received in
custody awaiting transportation to, or arrives voluntarily to commence service
of sentence at, the official detention facility at which the sentence is to be
served.
(b) Credit for Prior Custody.— A defendant
shall be given credit toward the service of a term of imprisonment for any time
he has spent in official detention prior to the date the sentence commences—
(1) as a
result of the offense for which the sentence was imposed; or
(2) as a
result of any other charge for which the defendant was arrested after the
commission of the offense for which the sentence was imposed;
that has not been credited against another sentence.