In an operating system, the time (clock) is determined by four parts: time value, time standard, time zone, and Daylight Saving Time (DST) if applicable.
After boot-up has completed, the system clock runs independently of the hardware clock.
The Linux kernel keeps track of the system clock by counting timer interrupts.
The localtime standard is dependent on the current time zone, while UTC is the global time standard and is independent of time zone values.
Though conceptually different, UTC is also known as GMT (Greenwich Mean Time).
An OS that uses the UTC standard will generally consider the hardware clock as UTC and make an adjustment to it to set the OS time at boot according to the time zone.