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from redhat documentation

The /etc/sysconfig/clock file controls the interpretation of values read from the system hardware clock.

UTC= where is one of the following boolean values:

true or yes — The hardware clock is set to Universal Time.

false or no — The hardware clock is set to local time.

so

regarding the UTC=true/false

please advice - what is the hw clock ?

I guess the hw clock is the BIOS clock of the red-hat machine but not sure ,

second what the impact when we change the hw clock ? why we need to change it?

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I recommend you read this article from RedHat.

Hardware Clock, is a clock that runs independently of any control program running in the CPU and even when the machine is powered off so it can keep track of the current time.

And hwclock is a utility for accessing the Hardware Clock - also referred to as the Real Time Clock (RTC) or CMOS clock. You can display the current time, set the Hardware Clock to a specified time, set the Hardware Clock to the System Time, and set the System Time from the Hardware Clock.

You can learn more about when we need to change it after understanding how to get accurate time by reading this article.

BTW, you do not have to use it on modern distributions, but knowing how it works and how it can be changed, might come handy one day.

A personal computer has a battery driven hardware clock. The battery ensures that the clock will work even if the rest of the computer is without electricity. The hardware clock can be set from the BIOS setup screen or from whatever operating system is running.

The Linux kernel keeps track of time independently from the hardware clock. During the boot, Linux sets its own clock to the same time as the hardware clock. After this, both clocks run independently. Linux maintains its own clock because looking at the hardware is slow and complicated.

The kernel clock always shows universal time. This way, the kernel does not need to know about time zones at all. The simplicity results in higher reliability and makes it easier to update the time zone information. Each process handles time zone conversions itself (using standard tools that are part of the time zone package).

The hardware clock can be in local time or in universal time. It is usually better to have it in universal time, because then you don't need to change the hardware clock when daylight savings time begins or ends (UTC does not have DST). Unfortunately, some PC operating systems, including MS-DOS, Windows, and OS/2, assume the hardware clock shows local time. Linux can handle either, but if the hardware clock shows local time, then it must be modified when daylight savings time begins or ends (otherwise it wouldn't show local time).

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The hardware clock is the real-time clock (these days part of the southbridge chip) the same subsystem also stores BIOS settings, and the hardware clock can often be set via the BIOS settings menus.

On linux the hwclock tool can be used to access the hardware clock from the command-line.

Using UTC for the hardware clock give more consistent results on systems that see multiple time-zones (eg portables, and systems subject to daylight-sabings)

the hardware clock is used to set the system time after a reboot, if you aren't using some sort of NTP cline to set the time it's the only time benchmark the system has.

usually one of the shutdown scripts will update the hardware clock from the system time

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