Tom Van Baak's site on precise time and frequency, covering atomic clocks, GPS timing, the Time Nuts community, and relativity experiments.
HN links it as a starting point for precise timekeeping, and for its clock experiments testing time dilation on mountains.
The timing output from a $5 gps receiver isn't particularly impressive. You have to go to pretty long time intervals for a non-timing GPS to win out. Depending on your application those intervals may be thoroughly irrelevant, e.g. using the device as a frequency source rather than a clock.
There is also, of course, the issue of infrastructure dependence. Particularly since wireless telephony has moved almost exclusively to GNSS time we're going to have a really bad time if kessler syndrome takes out the GNSS satellites.
Edit: Here is an example adev chart for a inexpensive atomic clock vs what appears to be a pretty good timing GPS receiver: https://www.thinksrs.com/images/instr/prs10/PRS10diag2LG.gif
So in that case the GPS accuracy only beats out the free running atomic clock at intervals greater than 200,000 seconds or so.
Here is a collection of older timing receivers: http://www.leapsecond.com/pages/3gps/gps-adev-mdev.gif
A quick look didn't turn up any cheap non-timing receivers, but my experience is they're pretty bad (I mean relative to atomic standards, of course).
There are better timing receivers than the ones charted above, of course, but they are not $5. Their cost is now in the same general ballpark as surplus atomic clocks.
Of course, if you have both you can sync one to the other with whatever time constant maximizes the composite performance and have the best of both.
(and primary atomic clocks don't have this drift issue, but sadly the days of…
Quartz clocks can be inaccurate and can vary greatly with temperature. Beyond temperature-compensated crystal oscillators (TCXO) which are designed to be reasonably stable over a prescribed temperature range, there are oven-controlled crystal oscillators (OCXO)[1], that are held at a stable temperature in a small can with a heating element controlled by something like PID control. For anything requiring affordable stable timekeeping (this includes high data rate/quantization multimeters like the HP 3458A), an OCXO is a pretty good way to go. GPS is a fine reference to start one from and with specialized hardware GPS itself can be used as a MHz clock. For everything else there are TCXOs, and if you really don't care on-die RC oscillators can be used (this is the internal oscillator on an Atmega, for example, the default until you add an external crystal). If money is no object, atomic clocks are of course also an option. Past press mentioned that Google uses GPS-referenced atomic clocks to keep their data enter clocks in sync [2].
As an amusing aside, my kitchen oven clock has a poorly-chosen TCXO, such that it is not stable over the temperature range normally experienced by an over control panel. It's a great example of not designing to the application. Whenever I bake something, time accelerates on the oven clock. It's like a cookie-making time machine.
If you are interested in learning more about the hardware of crystal oscillators, this application note from HP is…
> So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth?
From the photon's perspective, it passes earth immediately. From our perspective, it takes 7 or 8 minutes.
> And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey?
You would have aged as much as your watch says you would have aged. Zero seconds.
>If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth?
I don't know how to do the math, but a very, very long time would have passed on earth.
> And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible?
Yes, you would be younger than him, and it's possible because that's just how relativity and time dilation work. It's even practically measurable in "real" life: http://www.leapsecond.com/great2005/tour/
Eh, doing the math[0], it seems like you'll actually drift about 1 nanosecond every ~35 minutes if I pick a mountain near my house vs sea level. That means you'd have about 1 microsecond of difference in your clocks in about 3 weeks. For a 1 millisecond drift though, that'd take about 66 years. That's still pretty safe, but it's definitely not "you never have to worry about this˟" territory. I'm sure some others can chime in with cases where 1 microsecond clock drift (in cases expecting very accurate clocks) can cause problems.
>>> g = 9.8 c = 299_792_458
>>> h = 4392 # height of Mt. Rainier
>>> drift_per_second = (g*h)/(c**2)
>>> print(drift_per_second)
4.789023865263744e-13
>>> nanosecond = 1.0e-9
>>> seconds_to_drift_one_ns = nanosecond / drift_per_second
>>> print(seconds_to_drift_one_ns)
2088.1081993625176
>>> print(seconds_to_drift_one_ns / 60) # minutes till a 1 ns drift accumulates
34.80180332270863
>>> print((seconds_to_drift_one_ns * 1000) / 60 / 60 / 24) # days till ~1 microsecond accumulates
24.167918974103213
[0] - http://www.leapsecond.com/great2016a/#%3A~%3Atext%3DConceptually%2C%20the%20experiment%20is%20very%2Cthe%20clocks%20should%20still%20agree
See for example the experiment conducted by Tom Van Baak about 15 years ago: http://leapsecond.com/great2005/.
He drove three Cesium clocks up Mount Rainier and returned after a week. He compared them to a clock he left at home for the journey. The graph that he shows on the page and his associated commentary is interesting.
Ultimately, a clock is simply an abstract device that goes tick-tick-tick at some regular rate. Once one starts measuring the phenomenon itself---mechanics of the human heart, tidal forces on the Earth, friction in a pendulum, or the uncertainty principle in atoms---it is no longer feasible to treat it as objective time.
International Atomic Time (TAI) itself consists of an ensemble of 400 atomic clocks, with the collective being more accurate (the proper word is "stable", I think?) than any of its constituents.
Yes, the original Mt Rainier experiment in 2005 caught the attention of Stephen Hawking and I was asked to repeat the experiment for his PBS/BBC TV series in 2016. It was mid-winter up here in PNW so we used Mt Lemmon in Arizona instead:
http://www.leapsecond.com/great2016a/
And in 2018 I was asked to be part of a "time travel" episode on a History channel show. We used Palomar Mountain in California:
http://www.leapsecond.com/great2018a/
Each experiment was a little different; different combination of clocks, different audience, different mountain, different elevations, different latitude, etc.