System time is based on the POSIX time standard, where time is measured as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970. A day is defined as 86400 seconds and leap seconds are subtracted automatically.

Date and time representation on a system can be set to match a specific timezone. Zone information files are stored in /usr/share/zoneinfo. Typically, the zone files are organized in subdirectories named for each region. The following commands allow you to view all of the zone information files available:

# cd /usr/share/zoneinfo
# ls -lR

To set the system timezone for a particular zone, you must copy the appropriate zone file to overwrite /etc/localtime. It is important that you also update the /etc/sysconfig/clock file to match the timezone that you have set, so that other applications can determine which zone information file you are using. This is best achieved by editing the /etc/sysconfig/clock file and then running the tzdata-update command. For example:

# sed -i "\|ZONE=|c\ZONE=America/Los_Angeles" /etc/sysconfig/clock
# tzdata-update

Substitute America/Los_Angeles with a valid timezone entry. The setting takes effect immediately. Some long running processes that might use /etc/localtime to detect the current system timezone, may not detect a subsequent change in system timezone until the process is restarted.

Note that timezones are largely used for display purposes or to handle user input. Changing timezone does not change the time for the system clock. You can change the presentation for system time in any console by setting the TZ environment variable. For example, to see the current time in Tokyo, you can run:

# TZ="Asia/Tokyo" date

To set system time manually, you can use the date command. For example. you can run:

# date -s "2018-10-28 01:59:59"

This command sets the current system time based on the time specified assuming the currently set system timezone. The command does not update the system Real Time Clock (RTC). You can set the system RTC to match the current system date and time using the hwclock command. For example:

# hwclock --show
# hwclock --systohc --utc

Consider configuring your system to use network time synchronization for accurate time-keeping. This can be particularly important when setting up high-availability or when using network-based file systems. See Chapter 14, Network Time Configuration for more information on configuring network time services that use NTP.