start = datetime.now()
end = start + timedelta(seconds=60*15) # 15 minutes

random_unixtime = random.randrange(
                     int(start.strftime("%s")), 
                     int(end.strftime("%s"))
                  )

# or
import time
random_unixtime = random.randrange(
                     time.mktime(start.timetuple()), 
                     time.mktime(end.timetuple())
                  )


datetime.fromtimestamp(random_unixtime)