1. As a test of speed or time; in order to beat a speed record or time limit.
John ran around the track against time, because there was no one else to race against.
2. As fast as possible; so as to do or finish something before a certain time.
It was a race against the clock whether the doctor would get to the accident soon enough to save the injured man.
3. So as to cause delay by using up time.
The outlaw talked against time with the sheriff, hoping that his gang would come and rescue him.