[Lazarus] How to program with time in milliseconds?

[Junior]

Lazarus 1.3 r44997M FPC 2.6.4 i386-linux-gtk 2 - Ubuntu 14.04
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I need to develop a game for all possible Operating Systems.

It will be based on time. I will use hundreds of time.

I wanted some tips with low-level routines to improve performance.

I'll be using multimedia features (sound and images) and access to the 
server using any internet protocol.

I plan on using compiler directives for each operating system to use API 
to improve performance.


Thanks