What this tool does is modify the clock in GTA:SA to provide real time play. To use it simply run the tool, and then run GTA:SA, though it is possible to do it the other way round if you decide you want real time in the middle of your game.

It works by accessing the GTA:SA ram, disabling the games clock update routines, and setting the time accordingly. You can also adjust the timezone you want the game to be in.