You don't need to call GetGameTimeMilliseconds(). There are two args passed through to your OnUpdate function. The first is the frame that the OnUpdate is running for. The second is the game time in milliseconds.
/edit: re. another question... The event handler is what is getting/passing through the event code. This is so the handler knows which event was fired (and you can do a check to make sure it is the event you want, if using the same function for more than one event). But some events provide their own returns - these are what are listed in the docs.