Hwclock command on Linux (machine clock) [Basic Guide]

The hwclock command on Linux can be used to read the machine clock or update the machine clock with the system clock and vice versa.

As stated at the beginning of the chapter, computers maintain two clocks. A so-called machine clock (hardware), is turned on when the system is inactive, and is held by a quartz crystal and battery when the computer is turned off. This battery is capable of keeping the watch even when disconnected from the electrical network for a long time.

When the computer is turned on, Linux reads the machine clock to update its system clock. It is common for there to be a small difference between these two watches.

The hwclock command has the following options:

• -r: enter the hardware clock times with the local time zone;
• -s: set the system clock with the hardware clock times;
• -w: configure the hardware clock with the system clock times;
• -c: compares the hardware clock and system clock times every 10 seconds;
• –adjust: add or subtract time from the hardware clock to adjust it to frequency slippage;

Examples:

# hwclock -c 
hw-time system-time freq-offset-ppm tick 
1571594417 1571594417.011423 
1571594427 1571594427.012649 123 

1

To set the Hardware clock with the system clock:

# hwclock -w

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