Why do the clocks go back and does it mean time is changing?
This Sunday 25 October, clocks across the UK will go back by one hour. But what is the science behind this long-standing tradition?
3 minute read
Firstly, we should make it clear that while our clocks, watches and devices change twice a year, time itself does not. The change is so we can make better use of daylight. As the UK tilts away from the sun in autumn and winter we experience shorter days and longer nights, so seasonal clock changes help us align our daily routines with daylight hours.
Standard time in the UK is based on Coordinated Universal Time (UTC), which is maintained, in part, with incredibly precise atomic clocks here at NPL. During daylight savings, UTC itself remains unchanged – we simply add one hour to the standard time to get UTC+1 in spring and then remove that hour to take us back to UTC in autumn.
The UK first adopted daylight saving time in 1916. More than a century later, changing the clocks remains a familiar part of life but the science of timekeeping remains far less visible.
Maintaining the national time scale
When we all wake up on Sunday morning and adjust our wall clocks, how can we trust that it’s the time we think it is? As the birthplace of atomic timekeeping and the home of UK time, it’s our role to provide access to trusted and reliable time. The UK national time scale is maintained at NPL using a group of commercial atomic clocks and two caesium fountain primary frequency standards. Learn more about how this works here: https://www.npl.co.uk/research/time-frequency/time-scales
NPL disseminates time across the UK by two well-established methods: radio and internet. That’s how some of our home devices update their time automatically without manual involvement – radio-controlled clocks synchronise with our radio time service while computers and networks can synchronise their clocks using our Network Time Protocol (NTP) service. Both provide access to the time from NPL with an accuracy better than 10 milliseconds.
Why is precision so important?
As well as enabling us to tell the time of the day, accurate timekeeping is key to many of the digital technologies that we depend on, from satellite navigation systems and the internet, to timestamping for transactions in financial trading and energy infrastructure.
Even fractional errors or changes to these time signals can result in huge differences. For example, Global Positioning System (GPS) determines your location by measuring the time it takes signals to reach your device from satellites. These signals travel at the speed of light, so even 1 millionth of a second of error would mean your position is off by 300 metres.
Alongside using atomic clocks to ensure our time is accurate to within a few nanoseconds, NPL also runs a programme called the National Timing Centre that will deliver trusted national time to help strengthen resilience across essential services and digital infrastructure – because resilient, traceable time is the invisible utility behind modern life.
We have been measuring time for centuries and whether it was using sundials, pendulum clocks or our modern-day atomic clocks, this weekend’s clock change is a reminder that time is one of the most important measurements we make.
23 Oct 2026
When do the clocks change?
When do the clocks go back? NPL details UK Daylight Savings Time, if other countries change their clocks, and information about the role of atomic clocks.