Problem
Sometimes the time between events has significance (the SUT behaves differently depending on the timing)
Sometimes race conditions can cause unpredictable behavior
Symptoms
Look in the code for:
timeouts
timed sleeps
values or computations that depend on the time
How To Test
Generate inputs that are varied in time (i.e. some close together, some not close together).
From
Udacity's
Software Testing
Unit 1.