# Time critical events based on framerate

Problem Description

Basically, I've made a "programming game" in which I've made my own scripting language. It is a completely standard Lunar Lander game, though instead of directly controlling the lander using keyboard input, it instead fires a sequence of commands you have provided through the scripting language I made. This all works, and I've hit a critical problem.

So essentially, using the scripting language, you specify something like After 5 seconds, activate thrust for 10 seconds, and then after 2 seconds, rotate the lander for 4 seconds. The goal is then to chain together a sequence of commands which will get you to land the lander safely.

Of course, this comes with one problem. If you repeat the sequence of commands, each command will have to be fired at exactly the same spot in the level, that is, after x amount of seconds, but also with the lander being in the exact same spot. If you did not use timer-based movement, sure you could indicate that a command should activate after 5 seconds, but the lander moving at 60 FPS would have moved way further than a lander moving at 30 FPS.

Why of course, I thought I would simple just make the lander use timer-based movement so it is independent of FPS (I am using the very standard approach of taking the amount of time passed last frame, and multiplying it to a constant move speed). Then I would count seconds using the system clock. So the after x amount of seconds, the lander would be in the same spot in the level when the command activates.

Unfortunately, this approach do not work. Every time I run a sequence of commands, they will be activated with, sometimes very slight, variations, making it impossible to actually get a consistent and reliable path for the lander.

My Question

So my question is, am I tackling this problem a completely wrong way? What exactly do you do when it is absolutely critical that timed events defined by the user will activate at exactly the same time and spot in a level? I seem to be quite stuck with this question.

-
How are you measuring time? Are you certain that you are using a sufficiently accurate system for getting the time? – olevegard Jun 17 '13 at 20:09