Open Source Software and Linux

Nov 16 2008   10:12PM GMT

Set your hardware and system clocks with the date and hwclock commands

John Little Profile: Xjlittle

Today we’re looking at the date and hwclock commands to set your system and hardware clocks.

Have you ever put a server into production only to realize that you had not set the hardware clock to UTC or any other time? When you’ve already put the machine into production it’s pretty difficult to go to your manager and tell him that you need to shut the machine down to set the hardware clock. Using the date and hwclock commands will allow you to set both clocks with shutting down you machine.

Computer systems, administrators and users all rely on the correct date and time to function properly or make their jobs go smoother. The system uses cron to execute programs at a certain time. As an administrator you need the correct time when checking logs and looking for problems. If the system clock is off see how long it takes your users to start calling about incorrect dates and times on their documents. In short things are not going to go smoothly if the date and time is wrong on the system.

The most likely scenario is that your system clock is off because it is relying on your hardware clock for the correct time. Here is how we correct this. First you need to set the correct time for the system with the date command. Type the date command to see the date and time format of the string:

[root@virtual-host ~]# date
Sun Nov 16 16:51:53 EST 2008
[root@virtual-host ~]#

We want to use this exact type of string to set our system clock:

date --set=Sat Nov 15 18:49:00 EST 2008

Ok now we are going to set our hardware clock to this date:

hwclock --systohc --utc

That’s it. Now your hardware clock is set and your system clock can now keep the correct time by referring to it. You can also set your system clock from your hardware clock with the following command:

hwclock --hctosys

Both of these can be particularly important if you are trying to set up the ntp daemon to an internet time source and your system clock is off by more than a 1000 seconds. The ntp daemon will not work under these conditions so you must correct them.

Hope this helps you keep your systems on time!

-j

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