How do RTOS kernel keep track of time ?
Every kernel provides a mechanism to keep track of time. This is done by a hardware timer which interrupts the CPU periodically.
The ISR for this timer invokes a service provided by the kernel, which is responsible to update time dependent variables. This ISR is generally called a Clock Tick ISR. Kernel maintains a special list to keep track of tasks that are waiting for the time to expire. This is called delayed task list.