This variable will return whether the time line is looping (true) or not (false). You can change this variable to switch looping on or off and it works with a negative time line speed (if the time line position goes below 0 it will start again at the last defined moment).
For more information on Timelines see here.
if !timeline_loop timeline_loop = true;
The above code checks to see of the time line currently assigned to an instance will loop or not and if it doesn't it sets it to true.