Ubuntu – How to check if hardware clock is in UTC or Local time

15.10biosdual-boottime

From what I've read the hwclock -r command only returns the time in local time no matter how it is set. I think hwclock --utc or hwclock --localtime will change the clock to UTC or localtime. But I don't want to change it; I want to know how it is currently set.

So how can I check if my hardware clock is set to UTC time or local time?

Best Answer

Ubuntu 15.04 and later uses systemd as the default init system and includes the timedatectl program. Running timedatectl will output:

      Local time: Tue 2016-02-02 09:47:54 MST
  Universal time: Tue 2016-02-02 16:47:54 UTC
        RTC time: Tue 2016-02-02 16:47:54
       Time zone: America/Denver (MST, -0700)
 Network time on: yes
NTP synchronized: no
 RTC in local TZ: no

RTC time is the hardware clock. Compare it to the Local time and Universal times returned by timedatectl to determine how the hardware clock is set.

RTC in local TZ: no means the hardware clock is interpreted as UTC

RTC in local TZ: yes means the hardware clock is interpreted as local time

To switch using `timedatectl`

sudo timedatectl set-local-rtc 0 sets RTC to UTC

sudo timedatectl set-local-rtc 1 sets RTC to local time

Switch using config file

Setting the UTC= setting in /etc/default/rcS will change how the system interprets the hardware clock (UTC=yes or UTC=no).

Related Solutions

Ubuntu – How to migrate to UTC hardware clock

Edit the file /etc/default/rcS with your favorite text editor, ie:

sudo nano /etc/default/rcS

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Look for the UTC=foo (yes/no) line and change it to UTC=yes.

From the rcS(5) man page:

UTC
This is used to govern how the hardware real time clock is interpreted when it is read (e.g., at boot time, for the purpose of setting the system clock) and when it is written (e.g., at shutdown). If this option is set to no then the system clock is assumed to be set to local time. If the option is set to yes then the system clock is assumed to be set to something approximating Coordinated Universal Time (UTC). (POSIX systems keep a variant of UTC, without leap seconds.)

Ubuntu – How to set the clock in Windows to UTC / localtime

On my Windows 7 machine, the default is for HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\RealTimeIsUniversal to not exist. So, here are Windows instructions:

Hold down the Windows logo key and press R. There, type regedit. Click OK and if you're using Vista or newer, acknowledge the UAC warning.

Now, in the left pane, there is a tree view. Under Computer, there will be some things called HKEY_SOMETHING_OR_OTHER. You should click the + next to HKEY_LOCAL_MACHINE. Then, you should click the + next to SYSTEM to get ino HKEY_LOCAL_MACHINE\SYSTEM. Now, expand CurrentControlSet, then inside it, Control, and then TimeZoneInformation. Now, right-click the entry for RealTimeIsUniversal and select the option to delete it. Alternatively, you could set it's value to 0.