Unix systems and cousins traditionally store times as UTC, and convert them according to the local timezone settings only for display. On a typical Linux machine, /etc/timezone contains the time zone (such as Pacific/Auckland), and /etc/localtime is a SymLink to a binary file containing information on the standard and the daylight savings offset of that time zone, as well instructions on how to calculate when daylight savings is in effect. In the above-mentioned example it might look like this:

 $ ls -l /etc/localtime
 lrwxrwxrwx    1 root    root    33 Apr 22 00:19 /etc/localtime -> /usr/share/zoneinfo/Pacific/Auckland

The SuperUser can use tzselect(8) to change these settings. Make sure it is set correctly! If you have it set to UTC your system time will not reflect your localtime. ntpdate(1) will set the system clock correctly, but the hardware clock will get confused. You can compare these by looking at the output of date(1) and hwclock(8).

On Debian systems /etc/default/rcS contains a UTC variable that decides whether or not the hardware clock is set to UTC. Having it that way and setting UTC=yes is A Good Idea. The best reason you might want to use localtime for your hardware clock is if you dual boot with another OperatingSystem that fiddles with your hardware clock for daylight savings.

See UTC and NTP for more.

The internal representation of UTC time that all Unix systems use is often know as the Unix timestamp. This value is defined as the number of seconds since 00:00.00 01-01-1970. This value is always stored in UTC and when displayed is adjusted via your time zone to display your local time.

Try

 date +%s

or

 perl -e 'print time() . "\n"'

on a Linux box to see the current Unix timestamp.

 date --date="1970-01-01 <unix timetamp> secs UTC"

or

 perl -e 'print localtime( <unix timestamp> ) . "\n")

or

 perl -e 'print gmtime( <unix timestamp> ) . "\n") # for UTC

will give you a human readable time for a given timestamp.

In PHP/C/etc you can convert a time in localtime to a unix timestamp using the mktime(3) function. PHP Example below

 <?php
 $ts = mktime(11, 49, 00, 10, 17, 2004);
 echo strftime("%Y-%m-%d %H:%M:%S %Z<br>", $ts);
 putenv("TZ=UTC");
 echo strftime("%Y-%m-%d %H:%M:%S %Z<br>", $ts);
 ?>

If you run this script you'll get output like this (depending on the TimeZone of your server)

 2004-10-17 11:49:00 NZDT
 2004-10-16 22:49:00 UTC

A really simple way to find the current time/date somewhere to set the TZ variable to the timezone you're interested in and run date(1):

 TZ=Pacific/Auckland date

Excerpt from comp.risks 22.94

Motorola reports that several GPS receivers in its Oncore line will misdisplay the date on 28 Nov 2003 at midnight UTC. For a one-second window the receivers will mistakenly report the date as 29 Nov instead of 28 Nov.

Here's why. Every couple of years or so for the past three decades, the International Earth Rotation Service has announced a leap-second because the Earth is rotating slightly more slowly than an 86400-second day would suggest. But since 1 Jan 1999, we've had an unusually long dry spell without any leap seconds. The GPS week number in the UTC correction parameter is 8 bits long, which allows for 256 weeks of unambiguous time calculation. Until now this parameter has never rolled over, but because of the dry spell 28 Nov will be exactly 256 weeks after the most recent leap second, and the rollover will contribute to the bug. <http://www.motorola.com/ies/GPS/docs_pdf/notification_oncore.pdf>

Steve Allen writes in <http://www.ucolick.org/~sla/leapsecs/onlinebib.html> that some JDAM smart bombs and other munitions are rumored to contain these receivers. Anyone intending to use those weapons around the magic window might want to reschedule their bombing runs for some other time...

CategoryNotes