Use time.time instead of datetime.utcnow
Use time.time instead of datetime.utcnow

According to the document of time.time(), "Return the current time in
seconds since the Epoch" and "Seconds Since the Epoch" does not depend
on timezone [1].  So it's better to have less code to store the same
thing.  Also, IPython's wall time of %time is also calculated by
time.time().

[1] http://stackoverflow.com/a/20035913

tkf/compapp