# Topical for today: How do new clocks auto-sync to the correct time? **URL:** \ **Category:** Factual Questions\ **Created:** [March 12, 2017, 6:09pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005 "2017-03-12T18:09:17Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2017, 8:35pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/21 "2017-03-13T20:35:10Z")
> [@Lips\_Obsession](#): > > So are you saying the time is kept like a CPU clock, when it’s powered off? What if it was kept unplugged for several years? Would it still know the time, or given a dead battery would it not work? The manual says it has a lithium battery good for 5 years. Probably conservative. There is also a big section on how to set it if the battery dies. ---
**Author:** ![Shoeless](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shoeless/32/7099_2.png) [@Shoeless](https://boards.straightdope.com/u/Shoeless)\ **Post date:** [March 13, 2017, 8:54pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/22 "2017-03-13T20:54:42Z")
> [@beowulff](#): > > If I’m reading the [patent](https://www.google.com/patents/US6567344) correctly, this clock doesn’t actually get any time information from _any_ source. It has the UTC time programmed into it at the factory, and when the user configures it for his locale, it figures out everything. Kind of a cheat, if you ask me. Our clock radio in our bedroom is an Emerson, although not the same model as in Lips\_Obsession’s link. And that’s exactly how it works. The time is preset at the factory, and it has a battery backup to store it until you plug it in. There is a switch on the back to set it to one of four time zones to display the correct time. It also has a DST switch, which I have turned off because it looks like it was programmed around the old DST start & end dates. (I fake DST by just shifting the time zone ahead or back one.) ---
**Author:** ![Alley\_Dweller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alley_dweller/32/430_2.png) [@Alley\_Dweller](https://boards.straightdope.com/u/Alley_Dweller)\ **Post date:** [March 13, 2017, 9:15pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/23 "2017-03-13T21:15:45Z")
> [@ftg](#): > > Neither does GPS\* nor WWV. Such clocks have a time zone setting and have firmware that keeps track of the date and does auto-DST adjustments. (Until they change the dates again.). The WWV signal does contain a DST indicator in bits 57 and 58. Devices that get the time from WWV can and do use these bits to adjust for DST. [NIST Time and Frequency Services (NIST Special Publication 432) – pdf](https://www.nist.gov/document-5574) > [@](#): > > Daylight saving time (DST) and standard time (ST) information is transmitted at seconds 57 and 58. When ST is in effect, bits 57 and 58 are set to 0. When DST is in effect, bits 57 and 58 are set to 1. On the day of a change from ST to DST bit 57 changes from 0 to 1 at 0000 UTC, and bit 58 changes from 0 to 1 exactly 24 hours later. On the day of achange from DST back to ST bit 57 changes from 1 to 0 at 0000 UTC, and bit 58 changes from 1 to 0 exactly 24 hours later. And, FYI, the signal also contains leap second and leap year bits. ---
**Author:** ![Alley\_Dweller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alley_dweller/32/430_2.png) [@Alley\_Dweller](https://boards.straightdope.com/u/Alley_Dweller)\ **Post date:** [March 13, 2017, 9:21pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/24 "2017-03-13T21:21:05Z")
> [@beowulff](#): > > If I’m reading the [patent](https://www.google.com/patents/US6567344) correctly, this clock doesn’t actually get any time information from _any_ source. It has the UTC time programmed into it at the factory, and when the user configures it for his locale, it figures out everything. Kind of a cheat, if you ask me. Yes, I had a Sony clock that worked similarly. Note that these clocks don’t appear to adjust for leap seconds. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [March 13, 2017, 9:23pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/25 "2017-03-13T21:23:54Z")
> [@Shoeless](#): > > Our clock radio in our bedroom is an Emerson, although not the same model as in Lips\_Obsession’s link. And that’s exactly how it works. The time is preset at the factory, and it has a battery backup to store it until you plug it in. There is a switch on the back to set it to one of four time zones to display the correct time. It also has a DST switch, which I have turned off because it looks like it was programmed around the old DST start & end dates. (I fake DST by just shifting the time zone ahead or back one.) That’s what my Sony clock did, so it’s not all that accurate when you turn it on. Some TV channels used to broadcast a time signal (in NY, channel 13, the PBS channel, did) and I have a VCR that used to set itself. Now that those analog stations are all turned off, I think, I doubt that would still work. I doubt it’s using GPS, wi-fi, or anything like that. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 13, 2017, 9:35pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/26 "2017-03-13T21:35:40Z")
One radio time system that I’m surprised to have not seen built in to anything (except perhaps some cars) is RDS/RBDS over the FM band. This is a digital signal overlaid on analog FM radio and contains information like the name of the current song. It also contains the current time. A self-setting clock wouldn’t need to be programmed to the local stations; it could just scan through the FM band at the normal channel spacing, listening for a few tens of seconds to see if it got a correct time message. The protocol is simple and shouldn’t cost more than a dollar to build into a device, and would work anywhere that FM can penetrate (i.e., much better than GPS or NIST). Not all stations broadcast RBDS but you just need one. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [March 13, 2017, 9:49pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/27 "2017-03-13T21:49:41Z")
> [@Alley\_Dweller](#): > > The WWV signal does contain a DST indicator in bits 57 and 58. Devices that get the time from WWV can and do use these bits to adjust for DST. Good to know but that still leaves TZ setting which may or may not be smart enough to handle the places that don’t observe DST. And of course there’s the problem of states like Montana that are considering dropping DST. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [March 14, 2017, 9:12pm UTC](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005/28 "2017-03-14T21:12:58Z")
> [@CookingWithGas](#): > > It doesn’t carry information about what time it is–the clock just counts cycles and knows that 60 cycles is exactly one second (for the most part). To elaborate: The grid frequency is slightly low during peak demand times. The power utility counts cycles, and makes up the missing ones at night. They hit the correct number of cycles per day, and if they miss for some reason they fix it the next day. With computers, there is seldom a miss. Of course this is not true for outages. In the 1990’s I worked on a project for radio station automation. As part of that I was at a WKRP-like station in York, PA for a week here, and another week there. They carried some syndicated programs via a satellite dish on the roof. These started on-the-dot time wise, but the station had only a simple analog AC clock for the DJs to time the intros…which would drift during the day. The low tech solution was a sticky note with an arrow on it showing the position of the second hand at the top-of-the-minute. The DJ’s would track the satellite time hacks prior to the start of a sat. program, and move the sticky note around the clock as required. It would move within about a 15 second window over the day. [Previous page](https://boards.straightdope.com/t/topical-for-today-how-do-new-clocks-auto-sync-to-the-correct-time/782005.md?page=1)