7 ms·

There are two independent timekeeping authorities in the US, the NIST and the Navy, which maintain the official civil and military time respectively. By policy

There are two independent timekeeping authorities in the US, the NIST and the Navy, which maintain the official civil and military time respectively. By policy the civil and military time are synchronized, and the precise offset between civil and military time is monitored by the NIST as a form of quality assurance. The offset between the two is generally less than the error in the comparison method. The Navy offers a similar online time service although it is clearly far less developed from a design perspective (https://www.usno.navy.mil/USNO/time/display-clocks/simpletime https://www.usno.navy.mil/USNO/time/display-clocks/simpletim...). However, what's perhaps more useful is that the Navy continues to offer a telephone time service (202-762-1401) while NIST does not. Additionally, the GPS time, while operated by the Air Force, is precisely synchronized to the Navy time plus or minus a known offset (due to the decision to no longer apply leap seconds to the GPS constellation to avoid leap-second-related problems). There is some history here. The NIST is interested in timekeeping from a metrological perspective since it is important to many commercial and scientific pursuits. The Navy, on the other hand, has long been interested in accurate timekeeping because it is required for celestial navigation. The transit telescope maintained at the Naval Observatory comes from the legacy of the Naval Observatory as a tool to correctly adjust the clocks used by ships at sea for navigation. Similarly, many of the most significant advances in mechanical timekeeping were spurred by the need for compact and reliable clocks for use at sea. The Navy operates master clock equipment at Schriever AFB for closer proximity to the NIST equipment at Boulder, but the primary means of comparison of the two time bases is by observing the GPS time. Because of the simplicity and low cost of GPS time sources, GPS is as ubiquitous for time measurement as it is for navigation.

wglb 7y ago
> Navy continues to offer a telephone time service (202-762-1401) while NIST does not. But on this page https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv/telephone-time-day-service https://www.nist.gov/pml/time-and-frequency-division/radio-s... NIST shows a working phone number...
hamandcheese 7y ago
I like the Navy version far more.
dfee 7y ago
I tried calling the number but it just kept ringing.
- gilrain 7y ago
  Thus indicating the time: time for lunch.
dlgeek 7y ago
> However, what's perhaps more useful is that the Navy continues to offer a telephone time service (202-762-1401) while NIST does not. https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv/telephone-time-day-service https://www.nist.gov/pml/time-and-frequency-division/radio-s... (303) 499-7111 - WWV (Colorado) (808) 335-4363 - WWVH (Hawaii)
- sciurus 7y ago
  For those who don't know, the callsigns are because the audio used by the phone service is primarily used for broadcast on shortwave / HF radio. If you're in the US and have a shortwave receiver you can usually pick up the time broadcasts on one of their frequencies any time of day. https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv https://www.nist.gov/pml/time-and-frequency-division/radio-s...
walrus01 7y ago
One interesting thing about Schriever AFB and its critical role in the GPS system... It's one of the few US military facilities in the 48 states that has a true double perimeter fence system around it. Take a look at it from google earth in satellite/aerial view. The security around it is very nearly as high as the Pantex plant in Texas (nuclear bomb assembly/disassembly facility).
- throw0101a 7y ago
  > Take a look at it from google earth ... "Sputnik Street"?
  - michaelhoffman 7y ago
    The launch of Sputnik 1 by the USSR was the single most important driver of U.S. developing its own space activities. Makes sense to commemorate that.
    - OnACoffeeBreak 7y ago
      Thank you for not saying "by the Russians". It's a pet peeve of mine when Russia is used as a proxy for the USSR while the USSR was still intact.
      - deleted 7y ago
        [deleted]
      - dTal 7y ago
        Why? The USSR was, practically speaking, a Russian empire.
        OnACoffeeBreak 7y ago
        Having grown up in the USSR (not that this fact necessarily gives me any special points in this discussion), I simply look at it like this: a lot more nationalities contributed to accomplishments and failures of the USSR than Russians. For example, it's not just the Russians that helped defeat Germany in WWII, and it's not just the Russians that made Chernobyl (in Ukraine, BTW) the disaster that it became.
fyrabanks 7y ago
Thank you very much; this is a very detailed post that covers several things I was wholly unaware of. One minor gripe: > However, what's perhaps more useful is that the Navy continues to offer a telephone time service (202-762-1401) while NIST does not. NIST also has a phone # service, but regardless, I'm struggling to think of a scenario where dialing a phone # would be more useful and/or accurate than any other method. I haven't called a number to set my clock since the '90s.
- ignoramous 7y ago
  In the 2000s, we set clock from cable TV news channels that displayed time on-screen.
  - mikeryan 7y ago
    In the 80s we called POPCORN (767-2676) to get the current time. https://www.theatlantic.com/technology/archive/2016/06/remember-when-you-could-call-the-time/488273/ https://www.theatlantic.com/technology/archive/2016/06/remem...
    - dylan604 7y ago
      In the 80s it was also very common for local banks to provide a number that would provide time. It was also common for the sign outside the bank to have a clock attached as well. "At the tone the time will be..."
      - jcrawfordor 7y ago
        A somewhat surprising number of these services still exist, mostly just because it's very inexpensive to offer them now.
    - jrnichols 7y ago
      I remember doing this a lot as a kid! and was sad to see so many of them going away. https://www.latimes.com/archives/la-xpm-2007-aug-29-fi-lazarus29-story.html https://www.latimes.com/archives/la-xpm-2007-aug-29-fi-lazar...
js2 7y ago
NIST also operates WWV which broadcasts the time. https://en.wikipedia.org/wiki/WWV_(radio_station) https://en.wikipedia.org/wiki/WWV_(radio_station) https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv https://www.nist.gov/pml/time-and-frequency-division/radio-s... When I was a kid my dad had built a Heathkit multiband radio receiver[1] and we used that station to set our clocks at home. 1. https://commons.wikimedia.org/wiki/File:Vintage_Heathkit_16-Transistor_Multi-Band_Radio,_model_GR-43_(8340957439).jpg https://commons.wikimedia.org/wiki/File:Vintage_Heathkit_16-...
dhimes 7y ago
So if we're using the Nautical Almanac for navigation purposes,does it matter if we use NIST or Navy? Or is the synchronization very close? Also, where can I learn more? In the sailing community these questions come up, believe it or not.
euler_angles 7y ago
I am curious, when did the decision to make GPS not follow leap seconds occur? had you asked me before I read your comment, I would have said that it doesn't honor leap seconds and never has.
- Rebelgecko 7y ago
  Atomic clock timing services that existed before GPS didn't use leap seconds either. GPS time is related to TAI time, which is based on absolute elapsed time so it slowly drifts away from UTC when leap seconds get added. IIRC, GPS time is just TAI/TT time with a different epoch.
- timeinput 7y ago
  You're correct it never has. GPS time started 6 Jan 1980 and has ignored every leap second since.
  - tyldum 7y ago
    The painful fact is that it does include the leap seconds before January 6th 1980. Makes it messy to convert between UTC and GPS time. I think all the online converters I found a couple of years ago failed to get this right.
- jcrawfordor 7y ago
  Yeah, so GPS was designed to not apply leap seconds from the start, which is important because there's a field built into the protocol to send the current offset. The reason I describe it as "no longer applying" is because the GPS time is monotonic from UTC at an arbitrary starting point, so it doesn't match up with any of the strictly monotonic time bases either - that is, GPS time is monotonic but does not match the agreed upon international monotonic time standard, TAI, because GPS time has some of the UTC leap seconds applied. TAI is also, of course, fundamentally just a divergence from UTC caused by not applying leap seconds. However, formally, UTC is defined as TAI plus or minus the leap second offset in order to minimize divergence from UT1 (which is a smoothed astronomical time base rather than monotonic). GPS time is TAI plus or minus a different offset caused by some of the leap seconds being considered. There is a basis for this, LORAN did the exact same thing but with an earlier divergence point.
guscost 7y ago
If the history of timekeeping for navigation sounds interesting, check out the book (and movie) “Longitude” by Dava Sobel. It’s an incredible and largely-forgotten story.
anonymousiam 7y ago
NIST has offered telephone time for as long as I can remember (40+ years). You can reach it at: 303-499-7111
Something1234 7y ago
The phone number doesn't work. Just tried calling it on my cell phone. I'm just getting a constant ringing with nothing picking up I'm very disappointed.
- figgis 7y ago
  Worked for me just now. It clicks for a bit than says "US Naval Observatory Master Clock" + the time.
  - Something1234 7y ago
    For me it was ringing for 2 minutes before I gave up. I have the timestamp on my phone to prove it. But then again Sprint may just be trash at providing phone service yet again. I may try again from a VOIP provider later.

wglb 7y ago

> Navy continues to offer a telephone time service (202-762-1401) while NIST does not. But on this page https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv/telephone-time-day-service https://www.nist.gov/pml/time-and-frequency-division/radio-s... NIST shows a working phone number...

hamandcheese 7y ago

I like the Navy version far more.

dfee 7y ago

I tried calling the number but it just kept ringing.

gilrain 7y ago
Thus indicating the time: time for lunch.

dlgeek 7y ago

> However, what's perhaps more useful is that the Navy continues to offer a telephone time service (202-762-1401) while NIST does not. https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv/telephone-time-day-service https://www.nist.gov/pml/time-and-frequency-division/radio-s... (303) 499-7111 - WWV (Colorado) (808) 335-4363 - WWVH (Hawaii)

sciurus 7y ago
For those who don't know, the callsigns are because the audio used by the phone service is primarily used for broadcast on shortwave / HF radio. If you're in the US and have a shortwave receiver you can usually pick up the time broadcasts on one of their frequencies any time of day. https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv https://www.nist.gov/pml/time-and-frequency-division/radio-s...

walrus01 7y ago

One interesting thing about Schriever AFB and its critical role in the GPS system... It's one of the few US military facilities in the 48 states that has a true double perimeter fence system around it. Take a look at it from google earth in satellite/aerial view. The security around it is very nearly as high as the Pantex plant in Texas (nuclear bomb assembly/disassembly facility).

throw0101a 7y ago
> Take a look at it from google earth ... "Sputnik Street"?
- michaelhoffman 7y ago
  The launch of Sputnik 1 by the USSR was the single most important driver of U.S. developing its own space activities. Makes sense to commemorate that.
  - OnACoffeeBreak 7y ago
    Thank you for not saying "by the Russians". It's a pet peeve of mine when Russia is used as a proxy for the USSR while the USSR was still intact.
    - deleted 7y ago
      [deleted]
    - dTal 7y ago
      Why? The USSR was, practically speaking, a Russian empire.
      - OnACoffeeBreak 7y ago
        Having grown up in the USSR (not that this fact necessarily gives me any special points in this discussion), I simply look at it like this: a lot more nationalities contributed to accomplishments and failures of the USSR than Russians. For example, it's not just the Russians that helped defeat Germany in WWII, and it's not just the Russians that made Chernobyl (in Ukraine, BTW) the disaster that it became.

fyrabanks 7y ago

Thank you very much; this is a very detailed post that covers several things I was wholly unaware of. One minor gripe: > However, what's perhaps more useful is that the Navy continues to offer a telephone time service (202-762-1401) while NIST does not. NIST also has a phone # service, but regardless, I'm struggling to think of a scenario where dialing a phone # would be more useful and/or accurate than any other method. I haven't called a number to set my clock since the '90s.

ignoramous 7y ago
In the 2000s, we set clock from cable TV news channels that displayed time on-screen.
- mikeryan 7y ago
  In the 80s we called POPCORN (767-2676) to get the current time. https://www.theatlantic.com/technology/archive/2016/06/remember-when-you-could-call-the-time/488273/ https://www.theatlantic.com/technology/archive/2016/06/remem...
  - dylan604 7y ago
    In the 80s it was also very common for local banks to provide a number that would provide time. It was also common for the sign outside the bank to have a clock attached as well. "At the tone the time will be..."
    - jcrawfordor 7y ago
      A somewhat surprising number of these services still exist, mostly just because it's very inexpensive to offer them now.
  - jrnichols 7y ago
    I remember doing this a lot as a kid! and was sad to see so many of them going away. https://www.latimes.com/archives/la-xpm-2007-aug-29-fi-lazarus29-story.html https://www.latimes.com/archives/la-xpm-2007-aug-29-fi-lazar...

js2 7y ago

NIST also operates WWV which broadcasts the time. https://en.wikipedia.org/wiki/WWV_(radio_station) https://en.wikipedia.org/wiki/WWV_(radio_station) https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwv https://www.nist.gov/pml/time-and-frequency-division/radio-s... When I was a kid my dad had built a Heathkit multiband radio receiver[1] and we used that station to set our clocks at home. 1. https://commons.wikimedia.org/wiki/File:Vintage_Heathkit_16-Transistor_Multi-Band_Radio,_model_GR-43_(8340957439).jpg https://commons.wikimedia.org/wiki/File:Vintage_Heathkit_16-...

dhimes 7y ago

So if we're using the Nautical Almanac for navigation purposes,does it matter if we use NIST or Navy? Or is the synchronization very close? Also, where can I learn more? In the sailing community these questions come up, believe it or not.

euler_angles 7y ago

I am curious, when did the decision to make GPS not follow leap seconds occur? had you asked me before I read your comment, I would have said that it doesn't honor leap seconds and never has.

Rebelgecko 7y ago
Atomic clock timing services that existed before GPS didn't use leap seconds either. GPS time is related to TAI time, which is based on absolute elapsed time so it slowly drifts away from UTC when leap seconds get added. IIRC, GPS time is just TAI/TT time with a different epoch.
timeinput 7y ago
You're correct it never has. GPS time started 6 Jan 1980 and has ignored every leap second since.
- tyldum 7y ago
  The painful fact is that it does include the leap seconds before January 6th 1980. Makes it messy to convert between UTC and GPS time. I think all the online converters I found a couple of years ago failed to get this right.
jcrawfordor 7y ago
Yeah, so GPS was designed to not apply leap seconds from the start, which is important because there's a field built into the protocol to send the current offset. The reason I describe it as "no longer applying" is because the GPS time is monotonic from UTC at an arbitrary starting point, so it doesn't match up with any of the strictly monotonic time bases either - that is, GPS time is monotonic but does not match the agreed upon international monotonic time standard, TAI, because GPS time has some of the UTC leap seconds applied. TAI is also, of course, fundamentally just a divergence from UTC caused by not applying leap seconds. However, formally, UTC is defined as TAI plus or minus the leap second offset in order to minimize divergence from UT1 (which is a smoothed astronomical time base rather than monotonic). GPS time is TAI plus or minus a different offset caused by some of the leap seconds being considered. There is a basis for this, LORAN did the exact same thing but with an earlier divergence point.

guscost 7y ago

If the history of timekeeping for navigation sounds interesting, check out the book (and movie) “Longitude” by Dava Sobel. It’s an incredible and largely-forgotten story.

anonymousiam 7y ago

NIST has offered telephone time for as long as I can remember (40+ years). You can reach it at: 303-499-7111

Something1234 7y ago

The phone number doesn't work. Just tried calling it on my cell phone. I'm just getting a constant ringing with nothing picking up I'm very disappointed.

figgis 7y ago
Worked for me just now. It clicks for a bit than says "US Naval Observatory Master Clock" + the time.
- Something1234 7y ago
  For me it was ringing for 2 minutes before I gave up. I have the timestamp on my phone to prove it. But then again Sprint may just be trash at providing phone service yet again. I may try again from a VOIP provider later.