title: Subtracting two timestamp arrays
body: <p>I have two numpy arrays I created which hold a number of timestamps. The timestamps are in month,day,year,hour,sec format (eg. 12/8/2009 10:00) and I hope to use them to calculate speed. I have the speed function almost finished, I just cannot figure out how to be able to subtract the two arrays to find the difference between them. I tried using np.subtract..</p>
  
  <pre><code>def speedofelephant(lat1, long1, time1, lat2, long2, time2):
      distance = haversine_distance(lat1, long1, lat2, long2)  # meter
  
      delta_time = np.subtract(time1,time2)
  
      print delta_time
  
      # set speed
      speed = (distance / delta_time) # speed in m/s
      speed = speed * 3.6 # speed in km/h
  </code></pre>
  
  <p>But I get this error...</p>
  
  <pre><code>NotImplemented
  
  Traceback (most recent call last):
    File "C:/script.py", line 187, in &lt;module&gt;
      speed=speedofelephant(lat1, long1, time1, lat2, long2, time2)
    File "C:/script.py", line 182, in speedofelephant
      speed = (distance / delta_time) # speed in m/s
  TypeError: unsupported operand type(s) for /: 'float' and 'NotImplementedType'
  </code></pre>
  
  <p>Any suggestions?</p>