Britain’s famous Speaking Clock celebrates its 81st birthday on July 24, 2017 - and you can listen to its five voices by clicking on the video above.

Now a national institution and part of Britain’s heritage, the Speaking Clock was the first of the pre-recorded information services in the UK, provided through telephones.

[Read more: The fifth voice of the Speaking Clock is revealed]

Created for people who wanted to know the time and did not have a watch or clock to hand, the clock was initially only available in the London directory area, with the first British Speaking Clock introduced on July 24, 1936.

The Speaking Clock was designed and constructed at the Post Office Engineering Research Station at Dollis Hill in North London. The time announcements were automatically co-ordinated on the hour with Greenwich meantime signals.

In order to access the service, subscribers would dial the first three letters of the word ‘time’ as dials at the time included letters as well as numbers to aid automatic calls. Dialling T. I. M. led to its common name 'TIM'. The service went national six years later.

Customer information on the Speaking Clock

David Hay, head of BT Heritage, said: “The BT Speaking Clock is a national treasure. Even though we live in the digital age, more than 12 million calls are made each year to the BT Speaking Clock to get an accurate time check.

“Eighty years ago BT's technology created the Speaking Clock which remains a much loved part of British life today. The Speaking Clock has reached octogenarian status and celebrating its birthday demonstrates BT's determination to preserve the heritage of the world's oldest communications company.”

[Related story: Meet the Voices behind the Speaking Clock]

Jane Cain was the first voice, winner of a Post Office ‘Golden Voice’ competition, and used from 1936 until 1963. Pat Simmons, a London telephone exchange supervisor, became the second voice from 1963 until 1985. The third voice belonged to Brian Cobby who became the first male voice at 11am on April 2, 1985. An actor by profession before he joined BT as an assistant supervisor at a Brighton exchange, Brian was selected from 12 finalists in BT’s competition on December 5, 1984. Users who were around in the 1960s who listen hard enough might detect a familiarity – Brian was also the voice of “5-4-3-2-1 Thunderbirds are go!” in the famous Gerry Anderson TV series.

The fourth voice was Sara Mendes da Costa from Brighton & Hove. She became Speaking Clock voice at 8am on April 2, 2007. Sara won a BT competition during 2006 to find a new voice from the public, which had almost 18,500 entrants, simultaneously raising more than £200,000 for BBC Children in Need.

Sara Mendes da Costa, said: “I am very proud to be the fourth permanent voice for the Speaking Clock and have been since April 2, 2007, nearly ten years ago.”

The fifth and current voice was also the winner of a nationwide competition. Alan Steadman, a retired civil servant from Dundee, was announced on the BBC's The One Show in November 2016.

Originally the accuracy of the BT Speaking Clock was one-tenth of a second, but it is now accurate to within 30 microseconds.