Linux, Clocks, and Time

Article from LinuxSA

Introduction

This document explains how to set your computer's clock from Linux, how to set your timezone, and other stuff related to Linux and how it does its time-keeping.

Your computer has two timepieces; a battery-backed one that is always running (the hardware, BIOS, or CMOS clock), and another that is maintained by the operating system currently running on your computer (the system clock). The hardware clock is generally only used to set the system clock when your operating system boots, and then from that point until you reboot or turn off your system, the system clock is the one used to keep track of time.

On Linux systems, you have a choice of keeping the hardware clock in UTC/GMT time or local time. The preferred option is to keep it in UTC because then daylight savings can be automatically accounted for. The only disadvantage with keeping the hardware clock in UTC is that if you dual boot with an operating system (such as DOS) that expects the hardware clock to be set to local time, the time will always be wrong in that operating system.

Setting your timezone

The timezone under Linux is set by a symbolic link from /etc/localtime^[1] to a file in the /usr/share/zoneinfo^[2] directory that corresponds with what timezone you are in. For example, since I'm in South Australia, /etc/localtime is a symlink to /usr/share/zoneinfo/Australia/South. To set this link, type:

ln -sf ../usr/share/zoneinfo/your/zone /etc/localtime

Replace your/zone with something like Australia/NSW or Australia/Perth. Have a look in the directories under /usr/share/zoneinfo to see what timezones are available.

[1] This assumes that /usr/share/zoneinfo is linked to /etc/localtime as it is under Red Hat Linux.

[2] On older systems, you'll find that /usr/lib/zoneinfo is used instead of /usr/share/zoneinfo. See also the later section The time in some applications is wrong.

Setting UTC or local time

When Linux boots, one of the initialization scripts will run the /sbin/hwclock program to copy the current hardware clock time to the system clock. hwclock will assume the hardware clock is set to local time unless it is run with the --utc switch. Rather than editing the startup script, under Red Hat Linux you should edit the /etc/sysconfig/clock file and change the UTC line to either UTC=true or UTC=false as appropriate.

Setting the system clock

To set the system clock under Linux, use the date command. As an example, to set the current time and date to July 31, 11:16pm, type date 07312316 (note that the time is given in 24 hour notation). If you wanted to change the year as well, you could type date 073123161998'. To set the seconds as well, type date 07312316.30 or date 073123161998.30. To see what Linux thinks the current local time is, run date with no arguments.

Setting the hardware clock

To set the hardware clock, my favourite way is to set the system clock first, and then set the hardware clock to the current system clock by typing /sbin/hwclock --systohc (or /sbin/hwclock --systohc --utc if you are keeping the hardware clock in UTC). To see what the hardware clock is currently set to, run hwclock with no arguments. If the hardware clock is in UTC and you want to see the local equivalent, type /sbin/hwclock --utc

The time in some applications is wrong

If some applications (such as date) display the correct time, but others don't, and you are running Red Hat Linux 5.0 or 5.1, you most likely have run into a bug caused by a move of the time zone information from /usr/lib/zoneinfo to /usr/share/zoneinfo. The fix is to create a symbolic link from /usr/lib/zoneinfo to /usr/share/zoneinfo:ln -s ../share/zoneinfo /usr/lib/zoneinfo.

Summary

/etc/sysconfig/clock sets whether the hardware clock is stored as UTC or local time.
Symlink /etc/localtime to /usr/share/zoneinfo/... to set your time zone.
Run date MMDDhhmm to set the current system date/time.
Type /sbin/hwclock --systohc [--utc] to set the hardware clock.

Other interesting notes

The Linux kernel always stores and calculates time as the number of seconds since midnight of the 1st of January 1970 UTC regardless of whether your hardware clock is stored as UTC or not. Conversions to your local time are done at run-time. One neat thing about this is that if someone is using your computer from a different time zone, they can set the TZ environment variable and all dates and times will appear correct for their time zone.

If the number of seconds since the 1st of January 1970 UTC is stored as an signed 32-bit integer (as it is on your Linux/Intel system), your clock will stop working sometime on the year 2038. Linux has no inherent Y2K problem, but it does have a year 2038 problem. Hopefully we'll all be running Linux on 64-bit systems by then. 64-bit integers will keep our clocks running quite well until approximately the year 292271-million.

Other programs worth looking at

rdate - get the current time from a remote machine; can be used to set the system time.
xntpd - like rdate, but it's extremely accurate and you need a permanent 'net connection. xntpd runs continuously and accounts for things like network delay and clock drift, but there's also a program (ntpdate) included that just sets the current time like rdate does.