Prosecutors want the supermodel to say whether Taylor gave her a rough, or uncut, diamond as a gift in 1997.
The court issued the order forcing Campbell to testify after she avoided prosecutors for a year and made it known she had no wish to be part of the case.
Prosecutor Brenda Hollis said Thursday that Campbell will testify on Aug. 5 instead of July 29 as previously scheduled.
Taylor, the former president of Liberia, denies prosecutors' allegations that he provided arms and ammunition to brutal rebels during Sierra Leone's civil war in exchange for so-called blood diamonds.
