Yes, it *is* OS dependant, as I said on the first line of my reply. However you're wrong about the tick count, at least according to the Win32 manpage:
in reply to RE: (4) How can I keep track of the execution time of a script?
in thread How can I keep track of the execution time of a script?
Its resolution is limited, however it is in miliseconds.
[CORE] Returns the number of milliseconds elapsed since the last
system boot. Resolution is limited to system timer ticks (about 10
on WinNT and 55ms on Win9X).