i have begun creating a very simple game engine and i am trying to work out how to create a time scale for the game.by time scale i mean some way of increasing and decreasing the speed of the game(not the fps) like creating a slow motion effect ... i have no idea how this could be accomplished so any help would be appreciated. if this help im using windows, Opengl and C++ in the game engine...

How my engine is setup i have an update and draw function...update is called 25 times per second while display is called as much as possible.