Whether you're working with timers or simply getting the time of day, it's important that you understand how the OS works with time.
The first thing to consider is: what's a tick?
When you're dealing with timing, every moment within the microkernel is referred to as a tick. A tick is measured in milliseconds; its initial length is determined by the clock rate of your processor:
Programmatically you can change the clock period via the ClockPeriod() function.