Localtime vs GMT - what to choose?

474 · l33t Joined: 19 Apr 2002 Posts: 714

One thing I have never understood with Linux is what to choose when you get asked if the clock stores time in localtime or GMT (in rc.conf and kernel setup if I remember correctly). I have yet to see any documentation on it. I have always thought it has something to do with the CMOS clock on the motherboard, but I'm not sure. Please, can anyone enlighten me so I know the correct setting?

pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

Unless someone else can say otherwise, I don't think there's a good reason for the home user to not choose local.
I had a job where Sun systems were all set to GMT in different locations (states and countries). This was so everyone
was using the "same clock". I'm not sure it really matters though. I have mine set for local and haven't had any
problems. As a side note, whatever Red Hat 7.1 does for time, it never matched when I booted to MS Windows. The
times differed by 6 or so hours and I could never get them to sync (not that I tried much).

AnimalMachine · Posted: Wed May 15, 2002 2:06 pm Post subject:

The choice is there mainly because GMT is the unix way to go ... but all those dual booters want to have their time stored local so Windows doesn't freak out.

... Or atleast that's my take on the situation.

pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

tomte · Tux's lil' helper Joined: 08 May 2002 Posts: 122

barry · Apprentice Joined: 01 May 2002 Posts: 170 Location: UK

The BIOS stores the time and date information without any concept of timezone. You either set this date to your local time or to GMT (aka UTC).

If you set it to GMT, then Linux automatically will add/subtract the necessary number of hours, taking daylight saving into account. This is the cleaner way of doing things.

Windows assumes your BIOS is set to local time and will physically change the date stored in the hardware. If you dual boot with Windows, you need to set it to localtime.

pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

Good answers... makes sense.

rac · Posted: Tue Feb 04, 2003 11:31 pm Post subject:

Another reason for people in locales that use daylight savings time to select GMT is that it prevents "clock warp", which can mess up all kinds of things, notably make. If you untar something during the daylight savings switchover hour, make may refuse to work after the switch for an hour, because it will think timestamps are in the future. In general, the system clock should not have discontinuities, IMHO. Timezone display should be an output thing, not an internal thing. I recommend "GMT".
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pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

With GMT, keep in mind that MS Windows will be 1 hour off (or linux if you set MS to the correct time). I've not read of a solution to this problem other than using "local".
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LosD · Posted: Wed Feb 05, 2003 4:47 am Post subject:

chatgris · Guru Joined: 14 Oct 2002 Posts: 383 Location: Canada

pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

Not in my experience. With the timezone set correctly, the time is off by 1 hour under MS Windows. I suppose it could be something non-intuitive.
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goanuj · Posted: Mon Jun 23, 2003 7:15 pm Post subject: dual boot clock problem

Hi there,

DISCLAIMER: for work reasons I need win2k on my computer.!

Basically it looks like linux is mucking around with my clock on startup and exit.

Can someone who dual boots please help me out, I have read the posts, but I think it is time for a FAQ so that one does not have to set the time every time one boots into Win2k.

Questions:
1. Please tell me what CLOCK in /etc/rc.conf should be set to, according to the FAQ it should be set to "local" but I have seen my clock get messed up quite a bit because of that.

2. should I have a /etc/adjtime file?

3. Could there be something in my init or shutdown scripts that is really messing up the clock? where would I find it? I have tried (in /etc) to

LosD · Posted: Mon Jun 23, 2003 7:34 pm Post subject:

I dualboot... My CLOCK is set to "local", my /etc/localtime is symlinked to /usr/share/zoneinfo/Europe/Copenhagen... (Because I live in... Well, take a wild guess

)

I haven't messed with /etc/adjtime, but it's there, maybe created from the zoneinfo file? Dunno...

I dualboot as well, no problems with screwed up clocks here... Except for daylight saving change dates, as both Windows and Gentoo wants to update it, as far as I can remember...

Hope this helps!

Dennis

Naan Yaar · Bodhisattva Joined: 27 Jun 2002 Posts: 1549

1. It should be set to local.
2. adjtime will be created by the hwclock command. You can remove it, and before rebooting to Gentoo, set the correct hardware clock time.
3. The clock script is in /etc/init.d/clock.

mlsfit138 · Guru Joined: 20 Sep 2003 Posts: 406 Location: Washington

I don't use windows, but is it possible to set you hardware clock to gmt, and inform windows that you are using that setting, and have windows automatically its own clock w/o messing w/ the bios clock? it seems that would solve the problem, and i think it would pretty lame if windows didn't have a feature like that. any windows gurus?

devsk · Posted: Sat Nov 08, 2003 1:23 am Post subject:

Kesereti · Guru Joined: 07 Nov 2002 Posts: 520

Thunderer · n00b Joined: 03 Oct 2004 Posts: 4

I really don't know what to make of this. Experienced the "clock skew" error for the first time and it was also the first time (that I remember) I set the clock to "local" in rc.conf.

Now that I booted to WinXP the clock was 2 hours late.

I've installed Gentoo quite a few times and never had such problems with using UTC in rc.conf and /Europe/Helsinki in the zoneinfo.

Any help on how to recover from the clock skew error?

bulash · n00b Joined: 14 Apr 2004 Posts: 63

Hm, an old thread, but since it is linked from the faq...

You CAN actually make windows use UTC! See here for reference.

In order to get it working look for

bjlockie · Veteran Joined: 18 Oct 2002 Posts: 1186 Location: Canada

krinn · Watchman Joined: 02 May 2003 Posts: 7470

or just set time to local, bios to your localtime so windows will always keep the good time
and set your time under linux with ntp and disallow any hardware time settings (no clock->linux or linux->clock in rc.conf)

you'll end with always on time linux and more/less (as good as your bios could) windows time

mp342 · Apprentice Joined: 03 Aug 2010 Posts: 165 Location: France

According to the previous link, windows now can handle UTC (M$ needs 4 years !).