I use "fptime", while I should have used "GetMsCount".

One possible cross-platform solution for this (certainly not the only 
one, but probably fitting quite well here) would be to use function 
GetMsCount from unit Dos. Regardless of the exact value returned by 
this call, the difference of values provided by two calls to this 
function should always be equal to the amount of milliseconds between 
the two calls (or at least as close to this as the implementation for 
a particular platform allows).