Time is part of the Project Management Golden Triangle. I have blogged about the Golden Triangle before, but the concept is that you have Time, Scope and Quality, as three sides to a triangle and they all impact each other. If you extend one side of the triangle, one or both of the other two sides will be impacted.
Time is an important aspect of Project Management. People who understand scheduling will understand how simple and how complicated time planning can be. On larger projects, the project plan will be controlling many different work streams in parallel and the Project Manager needs to understand the impact of time on each of the work streams and resources.