» Features

What's the time now? Well, it depends


Publication Date : 11-10-2012


When someone tells you to turn up at, say, 5pm on the dot, you may want to ask him if he means Singapore standard time (SST) or global positioning system (GPS) time.

There is actually a difference between the two time standards used here: GPS time is 16 seconds faster than SST.

The gap between them will widen by about one minute every 60 to 90 years, according to scientists' estimates. Eventually, GPS time would be 1min 16sec faster.

The closure of SingTel's 1711 time-announcement service last Monday - which was on GPS time - has cast the spotlight on the two time standards.

Organisations and people here have synchronised their clocks with either SST or GPS time.

But only one standard will hold up in a court of law when a dispute arises - SST.

SST is maintained by the National Metrology Centre, a unit of the Agency for Science, Technology and Research, using an ensemble of atomic clocks.

The data is sent to the France-based International Bureau of Weights and Measures for generating the international time-scale called Coordinated Universal Time (UTC), which airlines and global broadcasters synchronise their clocks with.

Atomic clocks on board GPS satellites, operated by the United States government, provide the other time standard.

Since GPS time was introduced in 1980, it has leapt ahead of UTC by 16 seconds. The gap has come about because UTC is manually adjusted from time to time to align it to Earth's slowing rotation due to the Moon's effect on tides.

The use of GPS time is getting more prevalent as more people own smartphones which let users set the time by linking up directly with GPS satellites.

Major public services here that have set their clocks to GPS time include the Land Transport Authority for its Electronic Road Pricing (ERP) gantries and Changi Airport Group at its three terminals.

Users in the other camp include Singapore Airlines and broadcasters like BBC and Bloomberg, which have set their clocks to UTC to coordinate global flight paths and broadcast hours.

Dr Liu Yan Ying, a metrologist at the National Metrology Centre, said a lack of precise coordination could cause problems.

"Operations across global financial markets, international transport networks and IT security networks require precise time synchronisation. A difference of even one second may disrupt their operations," she added.

On July 1 this year, more than 400 Qantas flights were delayed by as long as two hours when the Amadeus online flight-booking system found its internal clock at odds with external ones.

Disputes may also arise from having differing time standards, said Dr Liu.

For instance, ERP gantries are based on GPS time. If motorists set their time to the slower SST, they may think they still have 16 seconds to reach a gantry before it starts operating, although in reality it has already started levying charges.

"It is technically possible that complainants will have a case," said lawyer Bryan Tan of Keystone Law Corp. "The Road Traffic Act did not specify the clock on which toll charges are based."


Mobile Apps Newsletters ANN on You Tube