In every system, there are two clocks which comes into action while setting the time, one is hardware clock and the other is linux (OS) clock. The hardware clock determines the system clock on system boot. While the system is running, changes to one of these doesn't affect the other. Normally you can follow any procedure, like first update one clock and then sync it with the other but it is always advised to first update the hardware clock and then let the linux clock sync it with it at the next reboot. Changing the system clock by using the date program on a running system could cause date discontinuities and consequently problems. Down here, I will be discussing both ways.
In a linux/unix system, the time is the number of seconds elapsed since midnight UTC on the morning of January 1, 1970, not counting leap seconds.
There are different ways and procedures to change timezones in different flavors of linux/unix (which i will explain later in this HowTo) but universla procedure to do it in all flavors is explained below: