you just put a battery in, and it sets itself to the correct time.

And it stays there, automatically, for a year or more.

So it can't be receiving signals from satellites,

there's no wifi connection,

there's nowhere near enough power in here for that.

Instead, it uses much older technology.

Here at the radio transmission station in Anthorn,

the National Physical Laboratory, NPL, broadcasts a time signal to the UK

in a simple enough format that

the cheap electronics inside this clock can understand it,

and run accurately for over a year on a single double-A battery.

And at NPL's base in London, I asked their team how it all works.

- What we have in Anthorn are some very high-end cesium atomic clocks,

like the ones we have down here.

We broadcast from there, we monitor the signals at NPL

and accordingly put out steers to those clocks to correct for any any change.

It's a very low frequency, 60 kilohertz,

it has very low susceptibility to atmospheric effects.

We steer from NPL based on measuring what Anthorn is broadcasting

at the level of nanoseconds.

That is a nanosecond, the distance light travels in one nanosecond, 30 centimeters.

All that is taken into account.

- So that seemed simple enough. The official timescale is kept in London.

I wasn't allowed anywhere near the official atomic clocks,

I don't even know if they're actually at NPL's headquarters or not,

they might be somewhere else. They told me nothing!

But up north here, this transmitter has separate atomic clocks,

about four hundred kilometres away from the base.

Or 1.4 light-milli-seconds away.

There's a light-speed delay, a latency, of about a thousandth of a second

between the transmitter here and the base in London.

Because the team at NPL know exactly what that delay should be,

they can issue corrections if these clocks start to drift,

even by nanoseconds, billionths of seconds.

Officially, the accuracy is much lower than that, of course,

partly because they keep a very wide safety margin,

and partly because I can introduce a one-nanosecond drift to this clock

by doing that.

- What is really important when you're transferring time is not about the latency,

but understanding the latency.

So you can take that out of the equation.

From the NPL, we disseminate to the UK over radio broadcast.

Essentially that offers the entire UK mainland

several milliseconds of traceability to UTC.

We also have a fibre-delivered service

for regulatory compliance in the finance sector,

guaranteed at the microsecond.

And that's fine, I can understand all that. But then that leads to another question:

how does NPL know if their clocks in London start to drift?

Sure, they've got many of them, they cross-check,

and a "second" in the 21st century is defined

by the number of vibrations of a cesium atom.

But still, they can't be perfect.

They're meant to show the international standad,

UTC, coordinated universal time,

so what if NPL's official source of timekeeping truth drifts from that standard?

- UTC or coordinated universal time, as the name suggests,

is coordinated globally, contributing data from multiple atomic clocks

to generate what is the global timescale, UTC.

What we have here at NPL is the UK's timescale, UTC(NPL).

We have a whole suite of atomic clocks,

cesium clocks, hydrogen masers, and a cesium fountain.

The Bureau of Weights and Measures based in France

take the data from the clocks around the globe,

and inform each of the national labs how offset it is from UTC.

- And that's where the buck stops. In Paris.

Unless you start getting into physics and relativity and light cones,

and that's way beyond me.

Why does it matter?

Well, first of all, finance and high-frequency trading.

But also: science.

Things like the Square Kilometre Array, a planet-wide array of radio telescopes

that's being planned and built now.

When they're listening for incredibly faint radio waves from space,

all the telescopes around the globe have to be synchronised

with that level of precision, or the whole experiment could fall apart.

So then, I had a final question:

when accurate timecode is being sent from so many places,

when the public can get time that's more than accurate enough

for almost every purpose from navigation satellites in space:

why is this signal still important?

And the team from NPL had a rather serious answer.

- Time really is an invisible utility.

It underpins our digital infrastructure,

whether it's the synchronization of the energy grid,

or the telecom networks, or finance.

We are so dependent on GPS and other constellations.

One of the problems we face is those signals are easily disrupted.

It's the equivalent of a light bulb on the moon.

One of the things we're doing at the NPL

is to ensure that the UK is resilient for the future.

- Standing by the ocean with a domestic appliance.

The weird thing is, it's not the first time.