Prosecutors want Campbell to testify over a diamond they allege former president Taylor gave her. She was due to appear in court on July 29, but asked for a later date.
A spokesman for the war crimes court said Monday judges had approved the request, adding that Campbell was now scheduled to give testimony on August 5.
The prosecution has said Campbell could provide material evidence to rebut Taylor's claims that he never possessed rough diamonds. They accuse Taylor of taking diamonds to South Africa to buy weapons, which he denies.
In January, prosecutors said that during a visit to South Africa in 1997, Taylor gave Campbell a large rough cut diamond after a dinner hosted by Nelson Mandela.
A spokeswoman for Campbell has said that Campbell was a witness who had only been asked to help clarify events, and is not accused of any wrongdoing.
On trial in The Hague, Taylor denies all 11 charges of instigating murder, rape, mutilation, sexual slavery and conscription of child soldiers during wars in Liberia and Sierra Leone in which more than 250,000 people were killed.
