Helen Bailey, 51, was found dead in a cess pit at her house in Royston, Hertfordshire, in July 2016. She had not been seen since 11 April.
Ian Stewart, her fiance, denies murder.
Edward Farmer initially told St Albans Crown Court he saw her in a car in Broadstairs, Kent, between mid-April and June, but later admitted the sighting could have been weeks earlier.
The jury heard he claimed to have seen Ms Bailey on Charlotte Street, near to her holiday home in the town, "driving what looked to be a Range Rover or that type of 4x4, a dark colour".
He said Ms Bailey was by herself, despite defence claims she had been kidnapped by two violent men who had taken her to Broadstairs.
When questioned about the date of the alleged sighting, Mr Farmer said it was "about five, maybe six weeks prior to the police interview [on 13 July]".
Ms Bailey was reported missing by Mr Stewart on 15 April.
Under cross-examination by the prosecution, Mr Farmer admitted the sighting could have been in March, weeks before the author was reported missing.
He also admitted he had "no reason at all" to think about the sighting until he was interviewed by a reporter from The Sun in June.
Mr Stewart, 56, of Baldock Road, Royston, Hertfordshire, is accused of drugging his wife-to-be before suffocating her and throwing her in the pit underneath the couple's garage.
He denies murder, preventing a lawful burial, fraud, and three counts of perverting the course of justice.
The trial continues.
A cafe owner has told a court he saw a children's author near her holiday home weeks after she was reported missing.