The speaking clock is 72 years old and has had four permanent voices
The voice of the speaking clock is to be replaced with an actress in the role of Tinker Bell from Peter Pan.
Callers who dial 123 for the BT service will hear the new voice for three months starting at 0100 GMT on Sunday.
The change marks the end of British Summer Time and is part of a deal with Disney to promote a film of the 1904 JM Barrie play due out on 10 November.
The deal will temporarily replace Sara Mendes da Costa with Mae Whitman who also plays Tinker Bell in the film.
David Rooney, curator of time keeping at the Royal Observatory Greenwich, told the BBC: "For me as a historian, the most important thing is that it's going to be just as accurate as it ever was.
"It's all about the accuracy. We take a long view of these things - the voices come and go, what endures is the accuracy and Greenwich Mean Time."
He noted other experiments with the speaking clock's voice, including Lenny Henry in 2003, and a schoolgirl called Alicia Rowlands in the same year.
Mr Rooney said: "In the first year, 1936, when it was only available to London, it had 20 million calls, even now it gets 80 million calls a year... we're familiar with it, it's part of our culture and heritage."
The speaking clock is 72 years old and has had four permanent voices since 1936, including the current time-teller, Sara Mendes da Costa, who won the role in a competition.
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