The Speaking Clock, which tells the time to thousands of callers each day, is celebrating its 70th birthday.
The service, which is available on dialling the number 123, was founded in 1936 and went nationwide in 1942.
The accuracy of the time given has improved from within one-tenth of a second to within five-thousandths of a second - or five milliseconds.
BT says the clock itself is "the size of a small suitcase", compared with the first clock which was about 8ft long.
In all, 70 million calls every year are made to the service.
Three people have lent their voices to give the time on the Speaking Clock. The present voice is Brian Cobby, who was an actor before he joined BT.
David Hay, head of BT Heritage, said: "The BT speaking clock is part of the British way of life and the celebration of its 70th birthday is in keeping with BT's heritage policy."
The first voice on the Speaking Clock was that of Jane Cain, a telephonist, who won a prize of 10 guineas (£10.50) in a competition to find the right voice.
The second voice used was that of Pat Simmons, who died in 2005 aged 85.
Her recorded voice announced the time on the BT service from 1963 until 1985.
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