The biggest mechanical clock in Asia, and the third-largest in the world, has kept the time on the Bund since 1928.

Wei Yunsi has spent a quarter of a century maintaining the Customs House clock tower, a city landmark.

Wei Yunsi has been climbing the spiral iron staircase to the clock tower at the top of the Customs House on the Bund for 24 years.

The 57-year-old is the fourth person to have held the job of making sure Asia's biggest mechanical clock keeps chiming every hour, as it has done since Jan 1, 1928, sending notifications of the correct time across the entire Bund in Shanghai.

The job involves winding the clock springs, lubricating the gears and repairing minor damage to the machinery.

"To me, this is no longer a job. It's also an obligation and a way of life," said Wei, who has to climb 117 stairs to wind the clock every three days.

The tower houses the world's third-largest clock and offers views over the Bund, river and city center.

The clock and bell mechanisms were built according to the design of Big Ben at the Palace of Westminster. The bells were cast by the John Taylor Bell foundry and the mechanism built by JB Joyce & Co in England before they were shipped to Shanghai in 1927.

Located at 31 degrees 14' (north) latitude and 121 degrees 29' longitude, the coordinates of the Customs House used to represent the center of the city in the 1920s.

"The biggest requirement for this job is a sense of responsibility - you've got to make sure nothing goes wrong with Shanghai's landmark," said Wei.

Over the years, he has watched sweeping changes overtake the city from what once ranked as its highest building. Now it is eclipsed by skyscrapers on both sides of the Huangpu River, especially the cluster of iconic buildings in Lujiazui on the opposite bank.