Saturday, April 30, 2011

Linux Date Commands

Basic Command-line Tools:

The date command can be used as follows to display the time and date:

$ date
Fri Mar 28 16:01:50 CST 2003

To see UTC/GMT, you can do this:

$ date --utc
Fri Mar 28 08:04:32 UTC 2003

The date command also can be used to set the time and date. To set the time manually, do this:

# date -s "16:15:00"
Fri Mar 28 16:15:00 CST 2003

If you also need to adjust the date, and not just the time, you can do it like this:

# date -s "16:55:30 July 7, 1986"
Mon Jul 7 16:55:30 PDT 1986

There is also another way to set the date and time, which is not very pretty:

# date 033121422003.55
Mon Mar 31 21:42:55 PST 2003

The above command does not use the -s option, and the fields are arranged like this:
where MM = month, DD = day, hh = hour, mm = minute, CCYY = 4 digit year, and ss = seconds.

Please note that setting the clock with the date command must be done as root. This is a "savage" way to adjust the time. It adjusts the Linux kernel system time.

There is also a hardware clock (CMOS clock). You can look at the current hardware clock time with:

hwclock --show

I always keep my hardware clocks set to UTC/GMT. This maintains my clocks uniformly without any worries about "Daylight Savings Time". This is important, because when you set the hardware clock from the system clock (kept by the Linux kernel), you need to know if this is the case. To set the hardware clock from the system clock, leaving the hardware clock in UTC, enter the following:

# hwclock --systohc --utc
# hwclock --show
Fri 28 Mar 2003 04:23:52 PM CST -0.864036 seconds

Another interesting item is that the Linux system clock stores time in seconds since midnight on January 1st, 1970 (UTC). This is called UNIX time. Unfortunately, because this is a 32-bit value, there is a year-2038 problem. Hopefully, everyone will have moved to 64-bit architectures by then. In order to see the UNIX time, you can use the following command:

date +%s

Create a new soft link for /etc/localtime. Here is an example of step 3 and step 4:

# cd /etc
# ls -al localtime
lrwxrwxrwx 1 root root 39 Mar 28 07:00 localtime -> /usr/share/zoneinfo/America/Los_Angeles

# rm /etc/localtime

# ln -s /usr/share/zoneinfo/America/Denver /etc/localtime
# ls -al localtime
lrwxrwxrwx 1 root root 34 Mar 28 08:59 localtime -> /usr/share/zoneinfo/America/Denver

# date
Fri Mar 28 09:00:04 MST 2003

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