Linux, Clocks, and 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 initialisation 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”

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