UTC or GMT - which is mose accurate?

Stevekir

I have bought a good quality digital quartz watch and want to check its accuracy. Having set it exactly to a GMT signal on Worldtimserver.com and checking several times a day I notice that the watch changed from 0 (as set) to 1 second slow (varying over 5 to 10 minute periods) to now, 30 hours later it is 4 secs fast (varying over 5 minute periods by +-1 sec.) This variation is disappointing. However, I checked the accuracy of another, much cheaper, quartz digital watch and its timekeeping wandered in exactly the same way over the same 30 hours. It seems unlikely that both watches vary in exactly the same way..

So I wondered if the GMT signal as broadcast itself wavered. To check, I am now timing both watched on UTC as on its site on the Internet and so far the the above variation is absent. I think I will stick to UTC.

What do you think is going on please? Thanks.

unforeseen

UTC = GMT. Any discrepancies are due to delays in the method of transmission or similar and not the time itself.

You normally find that if you set to GMT your device will adjust itself for BST but not if UTC.

Stevekir

Thanks. I'm not using a device. I'm comparing the time (GMT or UTC) as shown on their sites, with the watch seconds hand as it goes round.


Therefore, can I assume that the UTC I am seeing is fully accurate, or is it also varying due to transmission time delays etc?


Thanks.

alanwsg

They'll both be subject to delays across the internet.

The most accurate time source the average person will have access to is a locked-on GPS receiver showing the time (The satellites have seriously accurate clocks on board).

Try downloading a GPS app that shows the time (e.g. GPS Test) and use the time displayed on that.

unforeseen

Stevekir wrote: »

Thanks. I'm not using a device. I'm comparing the time (GMT or UTC) as shown on their sites, with the watch seconds hand as it goes round.


Therefore, can I assume that the UTC I am seeing is fully accurate, or is it also varying due to transmission time delays etc?


Thanks.

You are not accessing the times directly. As has been suggested either a GPS receiver or a good quality radio clock (one where you can also set your distance from the time source) . Both of these will supply UTC

John_Gray

You might get a closer accuracy for a time signal by telephoning 123 from a landline. Using a mobile will include propagation delays...

John_Gray

unforeseen wrote: »

... a good quality radio clock (one where you can also set your distance from the time source) .

Please can you explain what difference the distance makes, given the speed of light?!

unforeseen

Sorry, I'm used to using it as a reference in radar systems where an error of just 10 microseconds gives a range error of one mile and also an error of a few milliseconds flags the return as stale.

System

John_Gray wrote: »

Please can you explain what difference the distance makes, given the speed of light?!

Even at the speed of light they travel 185 miles a second. If you're getting a time signal from WWV which is US based then you could be receiving the signal over 30 seconds after it has been transmitted.

Slithery

If you need an accurate clock then get a Linux machine running ntpd with a GPS receiver attached then sync all of your clients directly from it.

It's the only way to be sure...

tempus_fugit

Tarambor wrote: »

Even at the speed of light they travel 185 miles a second. If you're getting a time signal from WWV which is US based then you could be receiving the signal over 30 seconds after it has been transmitted.

I think you mean 185,000 miles a second. Light can get to the moon and back in just a few seconds.