Set Date And Time In Linux And Sync With Hardware (CMOS)

Why are the date and time important?

Having the correct date and time in your Linux box is very important. When you use emails ,the default time used by the application is the system calendar's date and time. Unless otherwise you have specified a different time scale. If you work in a client server network, most of the tasks are scheduled according to the system clock and the client synchronizes time with the server.

Interestingly, some people (including me) check the computer to find the date and time!
So, everybody should know how to change the date and time. For example, when you set up a new server and start business, you are not going to shutdown the server . What if you forget to change the date and time? The 'hwclock' command is there to save you. By using 'hwclock' you can change the time in the clock without shutting down the server.

Suppose, you are the network admin and you need to schedule a backup at mid night. If the system clock is not correct, you will end up running the backup at some other time and when you check the system logs you will go nuts!

Date command

To check the system date:

[[email protected] Desktop]$ date
Sun Jul 8 22:15:55 EDT 2012

How to check the date 2 days ago:

[[email protected] Desktop]$ date --date="2 days ago"
Fri Jul 6 22:17:58 EDT 2012

How to set the system date? ( I am going o change date to 2010-12-12 3.30p.m):

[[email protected] Desktop]# date '-s 2010-12-12 15:30:15'
Sun Dec 12 15:30:15 EST 2010
[[email protected] Desktop]# date
Sun Dec 12 15:30:17 EST 2010

Hwclock command

How to check CMOS date:

[[email protected] Desktop]# hwclock
Sun 08 Jul 2012 10:20:06 PM EDT -1.120674 seconds

Ok, now my system clock date is 2010 and CMOS date is 2012.Now I am going to change the CMOS date in order to synchronize it with my system date:

[[email protected] Desktop]# hwclock --systohc
[[email protected] Desktop]#

[[email protected] Desktop]# hwclock
Sun 12 Dec 2010 03:31:28 PM EST -1.499303 seconds
[[email protected] Desktop]#

Now, the CMOS clock date is set to 2010-12-12.I am going to change the system date to 2011-11-11.

[[email protected] Desktop]# date '-s 2011-11-11 15:30:15'
Fri Nov 11 15:30:15 EST 2011

Still my CMOS date shows 2010-12-12.I am going to change this to system date:

[[email protected] Desktop]# hwclock --systohc

[[email protected] Desktop]#
[[email protected] Desktop]# hwclock
Fri 11 Nov 2011 03:31:20 PM EST -0.449282 seconds
[[email protected] Desktop]#

Reverting back the changes:

[[email protected] Desktop]# hwclock --hctosys
[[email protected] Desktop]# date
Fri Nov 11 15:31:39 EST 2011
[[email protected] Desktop]#

What really happens

The computer's CMOS battery always synchronizes time with the system clock provided that the CMOS is working properly. You can use the 'hwclock' command to synchronize date manually.

Bobbin Zachariah 11:12 am

About Bobbin Zachariah

Founder of LinOxide, passionate lover of Linux and technology writer. Started his career in Linux / Opensource from 2000. Love traveling, blogging and listening music. Reach Bobbin Zachariah about me page and google plus page.

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