3.3. Using the hwclock Command
hwclockis a utility for accessing the hardware clock, also referred to as the Real Time Clock (RTC). The hardware clock is independent of the operating system you use and works even when the machine is shut down. This utility is used for displaying the time from the hardware clock.
hwclockalso contains facilities for compensating for systematic drift in the hardware clock.
hwclockutility saves its settings in the
/etc/adjtimefile, which is created with the first change you make, for example, when you set the time manually or synchronize the hardware clock with the system time.
hwclockcommand was run automatically on every system shutdown or reboot, but it is not in Red Hat Enterprise Linux 7. When the system clock is synchronized by the Network Time Protocol (NTP) or Precision Time Protocol (PTP), the kernel automatically synchronizes the hardware clock to the system clock every 11 minutes.
3.3.1. Displaying the Current Date and Time
hwclockwith no command line options as the
rootuser returns the date and time in local time to standard output.
--localtimeoptions with the
hwclockcommand does not mean you are displaying the hardware clock time in UTC or local time. These options are used for setting the hardware clock to keep time in either of them. The time is always displayed in local time. Additionally, using the
hwclock --localcommands does not change the record in the
/etc/adjtimefile. This command can be useful when you know that the setting saved in
/etc/adjtimeis incorrect but you do not want to change the setting. On the other hand, you may receive misleading information if you use the command an incorrect way. See the
hwclock(8) manual page for more details.
Example 3.9. Displaying the Current Date and Time
hwclockTue 15 Apr 2017 04:23:46 PM CEST -0.329272 seconds
3.3.2. Setting the Date and Time
--dateoptions along with your specification:
hwclock --set --date "dd mmm yyyy HH:MM"
--localtimeoptions, respectively. In this case,
LOCALis recorded in the
Example 3.10. Setting the Hardware Clock to a Specific Date and Time
rootin the following format:
hwclock --set --date "21 Oct 2016 21:17" --utc
3.3.3. Synchronizing the Date and Time
- Either you can set the hardware clock to the current system time by using this command:
hwclock --systohcNote that if you use NTP, the hardware clock is automatically synchronized to the system clock every 11 minutes, and this command is useful only at boot time to get a reasonable initial system time.
- Or, you can set the system time from the hardware clock by using the following command:
--localtimeoption. Similarly to using
LOCALis recorded in the
hwclock --systohc --utccommand is functionally similar to
timedatectl set-local-rtc falseand the
hwclock --systohc --localcommand is an alternative to
timedatectl set-local-rtc true.
Example 3.11. Synchronizing the Hardware Clock with System Time
hwclock --systohc --localtime