For more than a century, a point on the top of a hill in south-east London has been recognised as the centre of world time and the official starting point of each new day.
But now the supremacy of Greenwich Mean Time is being challenged by a gargantuan new clock being built in Mecca, by which the world's 1.5 billion Muslims could soon be setting their watches.
Due to start ticking on Thursday as the faithful fasting during the month of Ramadan, the timepiece sits atop the Royal Mecca Clock Tower which dominates Islam's holiest city.
It is at the heart of a vast complex funded by the Saudi government that will also house hotels, shopping malls and conference halls.
Bearing a striking resemblance to both St Stephen's Tower, which houses the bell of Big Ben, and the Empire State Building, the Saudi upstart aims to outdo its revered British rival in every way.
The clock's four faces are 151ft in diameter and will be illuminated by 2million LED lights along with huge Arabic script reading: "In the name of Allah". The clock will run on Arabia Standard Time which is three hours ahead of GMT.
When a glittering spire is added, topped with a crescent to symbolise Islam, the edifice will stand at nearly 2,000 ft, making it the world's second tallest building.
The clock of Big Ben, by comparison, is just 23ft in diameter, while its tower stands at a mere 316ft.
Residents of Mecca will also be reminded that it is time to pray when 21,000 green and white lights, visible at a distance of 18 miles, flash five times a day.
But Islamic scholars hope the clock's influence will stretch far further than the sands of Saudi Arabia, as part of a plan for Mecca to eclipse the Greenwich Observatory as the "true centre of the earth".
For the past 125 years, the international community has accepted that the start of each day should be measured the prime meridian, representing 0 degrees longitude, which passes through the Greenwich Observatory.
A standard time by which other clocks were set was needed to organise global travel and communications, but in the Islamic world the idea that it should be centred on a part of London is seen as a colonial anachronism.
As Mohammed al-Arkubi, manager of one of the hotels in the complex, put it: "Putting Mecca time in the face of Greenwich Mean Time. This is the goal."
According to Yusuf al-Qaradawi, an Egyptian cleric known around the Muslim world for his popular television show "Sharia and Life", Mecca has a greater claim to being the prime meridian because it is "in perfect alignment with the magnetic north."
This claim that the holy city is a "zero magnetism zone" has won support some Arab scientists like Abdel-Baset al-Sayyed of the Egyptian National Research Centre who says that there is no magnetic force in Mecca.
"That's why if someone travels to Mecca or lives there, he lives longer, is healthier and is less affected by the earth's gravity," he said. "You get charged with energy."
Western scientists have challenged such assertions, noting that the Magnetic North Pole is in actual fact on a line of longitude that passes through Canada, the United States, Mexico and Antarctica.