My phone was switched off since the past hour and I switched it on right now and I realised how normal it was for It to show me the exact present time and date and I wonder how.
In: Technology
Most devices like phones, computers and the like will have a number of independent sources for getting the correct time.
– an internal clock: That clock is not powered down when you power down the rest of the machine. It needs very little power to keep going and will have a tiny (backup) battery in some cases. Should you really not power the device for a very,very long time (or have the backup battery die or so): it’ll fail back to a start date -bad thing, see further.
– the GSM networks: the cell phone networks broadcast the time (relatively accurate) as part of their signal. A phone can detect these signals as soon as it power up its hardware – no need to actually be active on the network to get to basic information of any and all networks in the area. [I’m supposing CDMA networks work the same as GSM networks in the respect]
– GPS: the GPS/GLONASS/… satellites broadcast a signal from which an extremely accurate time can be derived (a very exact time is needed to use the positioning technology itself)
– NTP servers over the Internet. NTP (Network Time Protocol) is a way to get very accurate time by interacting with a (set of) distant server(s) even over a slow connection to still get a rather accurate time.
Most devices that care about having highly accurate time will know which sources give them the most accuracy and will use as many sources as possible and fallback/initialize time from a potential less accurate source first only to then refine later on as they can get access to more accurate signals as needed.
There are other less common sources of accurate time less likely to be used by phones: mainly radio sources broadcasting the time such as DCF77 and similar in the USA.
Why is it very important for modern computers to have a decent date/time idea – even when no software is one the device ?
The reason lies in security protocols such as SSL (the S in HTTPS e.g.): these are certificate based and certificates have a range of date/time on them when they are valid. Outside of that time they are invalid. That’s also why computer e.g. that have been put out of use for years often require getting the time/date set correctly before you can reinstall software onto them sometimes: they cannot validate the digital signatures if they don’t have a correct idea of the current date/time.
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