28 CFR § 2.10 - Date service of sentence commences.
(a) Service of a sentence of imprisonment commences to run on the date on which the person is received at the penitentiary, reformatory, or jail for service of the sentence: Provided, however, That any such person shall be allowed credit toward the service of his sentence for any days spent in custody in connection with the offense or acts for which sentence was imposed.
(b) The imposition of a sentence of imprisonment for civil contempt shall interrupt the running of any sentence of imprisonment being served at the time the sentence of civil contempt is imposed, and the sentence or sentences so interrupted shall not commence to run again until the sentence of civil contempt is lifted.
(c) Service of the sentence of a committed youth offender or person committed under the Narcotic Addict Rehabilitation Act commences to run from the date of conviction and is interrupted only when such prisoner or parolee:
(1) Is on court-ordered bail;
(2) Is in escape status;
(3) Has absconded from parole supervision; or
(4) Comes within the provisions of paragraph (b) of this section.