Daylight saving time and time knowing

  1. How does mobile phone or computer ‘‘know’’ what is the right time even after it has been shut down or the battery has been run out of energy?

  2. Today I turned the time on my mobile phone one hour back. But my computer already ‘‘knew’’ that it’s daylight saving time. How?

p.s. I do understand that primarily time is set up. I’m asking about the situation when it is done.

Computers have a small battery that keeps the clock functioning as well as storing some basic hardware data your computer uses to boot up.

As far as how your computer knew - daylight savings time switchovers happen at predetermined intervals, so it’s programmed into the clock function. Sometimes they do it on non-standard dates, so I’m guessing some background OS function fetches data about DST every once in a while to update the clock function.

I don’t think it is the BIOS that does the time changing, it is a function of the OS. For the last few days each time I clicked on the clock on my Vista machine there was a warning about the time change.

Also my Nokia E75 smartphone automatically changed the time.

GSM mobiles can receive time updates from the base station. This is an operator feature, and not all operators implement it. Not all of them get it right either. When we changed to summer time recently, some of my colleages reported that the time update was a day late in arriving. Probably because the actual time change happened in the early hours of Sunday morning. My carrier doesn’t provide time updates at all, but when I travel overseas my phone does receive them.

Unix systems use the ZIC time offset system. Unlike Windows, which uses local time for internal operations, Unix uses UTC. This is an important point. All operations that take a time tag are tagged with UTC, and are thus always correct, even if you move to a different time zone. However when you ask a Unix system to display a time, it will apply the currently in force timezone. This is actually an enviroment variable, which by default in inherited from the system’s master process (called init). You can override it if you wish to have some fun. Inside the Unix system config lives a set of files that define the timezone offsets for pretty much every place on the planet, and at pretty much every time in the last 100 years. When we say every time, we mean including all known daylight saving changes too. Also, if there is a change in the daylight saving dates or offsets anywhere, these changes usually come as part of a system update. The upshot is that you can take a Unix based computer to anywhere in world, and if you tell it where you are, it will have the correct local time, including daylight savings time.

It also means that a Unix system never sees time go backwards. (Not unless you deliberatly change its internal UTC base clock.) Most Unix systems when connected to the Internet will grab UTC from a NTP (network time protocol) server. This allows them to be accurate to fractions of a second. If a Unix machine is fast, the NTP system will slow down the internal clock until it is right, thus avoiding the system ever going back in time. On the whole it is all rather well worked out.

In Windows XP, just double-click on the time display in the lower right corner to open the Date & Time properties. Click on the Internet Time tab, and it will show whether your computer regularly updates the date and time from an Internet time server (default is Yes) and which server it uses (default is This also controls which time zone you are in and whether you want to adjust automatically for Daylight Saving Time. (I would guess that the PC or server can match the time zone to the user country selection for correctly setting this.) Other versions of Windows are similar.

If you never connect to the Internet, or to a network, then the BIOS keeps track of time internally. One of the clues that your PC’s BIOS battery had died used to be the startup date/time suddenly reverting to (IIRC) 00:00:00 Jan 01, 1980.

Thanks for giving me a heart attack - why didn’t I know that people elsewhere in the world change their times earlier? Is that a new thing?

It’s the other way around - it’s relatively new that the US and Canada change their times later!

Different countries have always done DST at different times of year. The US changed when we do it 3 years ago to go much later into November.

Sorry, yes, if it wasn’t clear I meant it was the OS that does the daylight time switching. I usually run dual boot system and both OSes will attempt to change the time back an hour, leaving me with the wrong time if I don’t correct it.

Nitpick. The US doesn’t go much later into November. The ending date was adjusted back one week from the last Sunday in October to the first Sunday in November. The stated reason for this (other than the energy saving) was to let Halloween trick-or-treating occur during DST. We did adjust the start date much earlier from the first Sunday in April to the Second Sunday in March.

The rule in European Union is the last Sunday in March to last Sunday in October. I don’t know if this is followed by all countries in the union though.

Right, it was the start of DST that they moved several weeks. It looks like it went from the first week in April to the second week in March, a much bigger change in the start time than the end time.

Same here. I have messed up DST before, and thought I’d f@cked up once more.

And of course, in Sydney, DST just began on Oct. 4. It will end on the first Sunday in April.

My husband and I once (before the US changed its start of DST) traveled to Italy at exactly the wrong time, late March. This resulted in us getting jet-lagged for the trip there from the US, then having to change times for DST, traveling back to the US and arriving on the Friday right before the US did their DST time change. :smack: That was not fun.

I always figured my computer was updating time from the web (windows XP), even if I set the clock manually, it checks with the local time it’s set to check and changes it back. (thanks to always-on DSL)

My phones have always updated from ATT’s network, too. I’ve never manually set a cell phone clock since the network went digital over 10 years ago, I’m pretty sure.

My over-the-air digital TV tuner also has a clock that updates from a source over-the-air. I have no idea what channel it gets it’s info from. I imagine it’s the same source that provides the programming guide accessed from the screen, but I don’t know what that is.

Oh, right, it’s from the invisible mother ship.


That change in when the US changes its clocks screwed up a lot of devices that are just smart enough to be able to keep track of the date and time (like VCRs) but have no way of learning from the outside world that we changed from “last Sunday in October” to “first Sunday in November.”

I guess it’s a good thing nobody uses VCRs anymore, huh? :slight_smile:

I missed daylight saving time?! I had no idea! I was wondering why it was getting dark on the way home from work. I was like “wow, I must have wasted more time than I thought on the computer, because it’s pitch dark outside now.”

DVD recorders have clocks, though.

dvd recorders have the new dst rules and selectable for which date sets to select

Not all of them. My old one switched on its own Sunday, and my newer one will do so next Sunday.