Basic Command-line Date 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: MMDDhhmmCCYY.ss 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

There are many useful formatting options for the date command. See the date manpage for details.

Of course, there is another useful tool available related to date and time: cal


$ cal -3
February 2003 March 2003 April 2003
Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa
1 1 1 2 3 4 5
2 3 4 5 6 7 8 2 3 4 5 6 7 8 6 7 8 9 10 11 12
9 10 11 12 13 14 15 9 10 11 12 13 14 15 13 14 15 16 17 18 19
16 17 18 19 20 21 22 16 17 18 19 20 21 22 20 21 22 23 24 25 26
23 24 25 26 27 28 23 24 25 26 27 28 29 27 28 29 30
30 31

You can also specify "cal -y" for the entire year, "cal" by itself for the current month, or "cal 12 2005" to see the calendar for December, 2005.


[edit]
Time Zone Configuration:

Background - The Earth is divided into time zones that are 15 degrees of longitude each, for this corresponds to the amount of angular distance the Sun appears to travel in 1 hour. 0 degrees longitude runs through the Royal Observatory in Greenwich, England. This is the origin of Greenwich Mean Time, or GMT. For all practical purposes, GMT and UTC are the same. To complicate matters, some countries observe Daylight Savings Time (DST), while others do not. Even within some countries, some states or districts do not observe DST while the rest of the country does! DST can also begin and end on different days in different countries! What a mess...

There are several files and directories that are used for time zones, and several tools:
/etc/sysconfig/clock - this is a short text file that defines the timezone, whether or not the hardware clock is using UTC,
and an ARC option that is only relevant to DEC systems.
/etc/localtime - this is a symbolic link to the appropriate time zone file in /usr/share/zoneinfo
/usr/share/zoneinfo - this directory contains the time zone files that were compiled by zic. These are binary files and cannot be
viewed with a text viewer. The files contain information such as rules about DST. They allow the kernel to convert UTC UNIX time
into appropriate local dates and times.
/etc/rc.d/rc.sysinit - This script runs once, at boot time. A section of this script sets the system time from the hardware clock and applies
the local time zone information.
/etc/init.d/halt - This script runs during system shutdown. A section of this script synchronizes the hardware clock from the system clock.
/etc/adjtime - This file is used by the adjtimex function, which can smoothly adjust system time while the system runs.
settimeofday is a related function.


redhat-config-date or dateconfig - These commands start the Red Hat date/time/time zone configuration GUI. Both commands failed to change the timezone in two different stock Red Hat 8.0 systems. They also failed to create a working ntp.conf file for the NTP server. The timezone problem went away after upgrading from the installed RPM, redhat-config-date-1.5.2-10, to a newer RPM from a Red Hat beta release, redhat-config-date-1.5.9-6.
zic - (The time zone compiler) Zic creates the time conversion information files.
zdump - This utility prints the current time and date in the specified time zone. Example:


# zdump Japan
Japan Sat Mar 29 00:47:57 2003 JST

# zdump Iceland
Iceland Fri Mar 28 15:48:02 2003 GMT

# zdump /usr/share/zoneinfo/Asia/Calcutta
/usr/share/zoneinfo/Asia/Calcutta Thu May 21 13:27:25 2009 IST

In order to manually change the timezone, you can edit the /etc/sysconfig/clock file and then make a new soft link to /etc/localtime. Here is an example of changing the timezone manually to "America/Denver":

1. Select the appropriate time zone from the /usr/share/zoneinfo directory. Time zone names are relative to that directory. In this case, we will select "America/Denver"

2. Edit the /etc/sysconfig/clock text file so that it looks like this:
ZONE="America/Denver"
UTC=true
ARC=false

Of course, this assumes that your hardware clock is running UTC time...

3. Delete the following file: /etc/localtime

4. 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