# How often do cell phones recheck the time?
**URL:** https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166
**Category:** Factual Questions
**Created:** [December 16, 2012, 6:59am UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166 "2012-12-16T06:59:14Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@Lord\_Il\_Palazzo](https://boards.straightdope.com/u/Lord_Il_Palazzo)
#### Post date: [December 16, 2012, 6:59am UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/1 "2012-12-16T06:59:14Z")
The title pretty much says it. If my cell phone somehow got its internal clock shifted and I’m in an area with good coverage, how long would it take to automatically correct? Is the rechecking continual or does it happen at a set interval?
Would the answer have been different older cellular technology? From five years ago? From ten?
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### Author:  [@voltaire](https://boards.straightdope.com/u/voltaire)
#### Post date: [December 16, 2012, 7:19am UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/2 "2012-12-16T07:19:13Z")
I’m sure someone much more knowledgeable will come around, but the way that cell phones are able to all simultaneously communicate within a given frequency is that they inherently and continuously keep track of the time. Pretty much all signals that go back and forth include a timestamp.
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### Author:  [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)
#### Post date: [December 16, 2012, 12:39pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/3 "2012-12-16T12:39:57Z")
It used to be that cell phones used analog signals and those signals were assigned communication channels by the cell tower when the phone set up a call. Each phone then completely hogged that channel until it was done talking. More modern digital protocols let phones share communication channels. In order to do that, the cell phone needs to know precisely what time it is so that it knows when the time has come for its turn to talk. A modern cell phone therefore has to know the time very precisely, where an older analog phone doesn’t need to know it at all.
Analog phone service died about ten years ago or so. Digital service has been around a lot longer than that, but up until that point it was still possible to use an old analog type cell phone.
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### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [December 16, 2012, 2:21pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/4 "2012-12-16T14:21:13Z")
I think they may check the time continuously, from any practical user perspective. But there is another issue: the time cell phone companies use can drift by seconds from other precise times such as GPS time or time per NTP. I have an iPhone app that uses NTP and shows the difference between cell phone time and the “real time”.
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### Author:  [@Musicat](https://boards.straightdope.com/u/Musicat)
#### Post date: [December 16, 2012, 2:36pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/5 "2012-12-16T14:36:18Z")
On several occasions, even though I am in tower range and getting a signal, I have noticed that the time shown on the phone is one hour off. It suggests that a correction for Daylight Saving Time has not been applied, or is mis-applied. It can persist for minutes (hours?) and eventually is corrected.
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [December 16, 2012, 2:37pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/6 "2012-12-16T14:37:55Z")
Sorry, but I don’t think the above answers are correct.
I have been working with an embedded GPRS receiver, and based on my experience, the only way to get the current UTC time is to register with the network. When this is done, if the modem is configured to output “WIND” responses, one of them will be the current network time, UTC offset, DST indication, carrier, etc. (e.g. - +WIND: 15,1,“T-Mobile”,2,“T-Mobile”,4,“09/08/12,15:32:28-20”,6,“1”)
Now, the cell phone software may be periodically re-registeing in order to get the time, but I don’t think this happens very often (maybe once every hour or so).
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### Author:  [@Digital\_is\_the\_new\_Analog](https://boards.straightdope.com/u/Digital_is_the_new_Analog)
#### Post date: [December 16, 2012, 2:46pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/7 "2012-12-16T14:46:54Z")
It’s still technology dependent. Some cell technologies effectively provide the time on every packet, and the phone can (and usually does) use that information.
Some do it much less often. Network Time is not required by all the standards.
It’s important to differentiate the ability of phones to be in sync on a clock from the ability for the phone to offset that clock to something useful for humans to reference.
In other words, modern phones need to know with high precision how much time they have until their next transmit slot, for instance. They don’t need to know the wall clock time of that transmit.
> [@Musicat](#):
>
> On several occasions, even though I am in tower range and getting a signal, I have noticed that the time shown on the phone is one hour off. It suggests that a correction for Daylight Saving Time has not been applied, or is mis-applied. It can persist for minutes (hours?) and eventually is corrected.
Sometimes, although likely not in the USA very often, the problem is that the network misconfigures it’s offset field. When network time is used, it’s based on GMT. The network tells the device how many hours off from GMT the current location is, and your phone does the math. A similar problem can be seen if you pick up a cell tower on the other side of a time zone change.
-D/a
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### Author:  [@Musicat](https://boards.straightdope.com/u/Musicat)
#### Post date: [December 16, 2012, 2:51pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/8 "2012-12-16T14:51:43Z")
> [@Digital\_is\_the\_new\_Analog](#):
>
> Sometimes, although likely not in the USA very often, the problem is that the network misconfigures it’s offset field. When network time is used, it’s based on GMT. The network tells the device how many hours off from GMT the current location is, and your phone does the math. A similar problem can be seen if you pick up a cell tower on the other side of a time zone change.
The offset error seems most likely in my case, as I’m not close to a time zone boundary.
However, what happens to those who straddle a zone line, and might get signals equally from towers on either side? Does a phone use GPS for correction (in the display) or just ignore what’s happening?
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### Author:  [@pwmeek](https://boards.straightdope.com/u/pwmeek)
#### Post date: [December 16, 2012, 3:45pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/9 "2012-12-16T15:45:02Z")
Mine usually only resets the time with an incoming call.
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### Author:  [@Digital\_is\_the\_new\_Analog](https://boards.straightdope.com/u/Digital_is_the_new_Analog)
#### Post date: [December 16, 2012, 7:17pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/10 "2012-12-16T19:17:58Z")
> [@Musicat](#):
>
> The offset error seems most likely in my case, as I’m not close to a time zone boundary.
>
> However, what happens to those who straddle a zone line, and might get signals equally from towers on either side? Does a phone use GPS for correction (in the display) or just ignore what’s happening?
I’m not sure..I’ll try to remember to ask tomorrow. I have a co-worker who dealt with this type of problem for a while.
-D/a
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### Author:  [@Kevbo](https://boards.straightdope.com/u/Kevbo)
#### Post date: [December 17, 2012, 5:46pm UTC](https://boards.straightdope.com/t/how-often-do-cell-phones-recheck-the-time/644166/11 "2012-12-17T17:46:01Z")
> [@engineer\_comp\_geek](#):
>
> Analog phone service died about ten years ago or so. Digital service has been around a lot longer than that, but up until that point it was still possible to use an old analog type cell phone.
Analog only completely phased out a couple years back. I was one of the holdouts using a tri-mode phone. This is because I specifically wanted a phone that would work in very remote areas, and analog persisted in remote areas for a long time because it covered more area with fewer towers, and the rural areas didn’t so much need the bandwidth.
When analog finally died, I finally got a smart phone. Wonderful where it works, but my very low population density state (NM) now has some big coverage holes in it that used to have decent analog signal. The timing required for digital imposes a strict 15 mile limit between tower and handset that antennas and boosters can’t do anything about.