fix for issue #14655

Fix for issue #14655

timedelta.microseconds returns the microseconds portion of the timestamp. Instead we call total_seconds() and the multiple by 1000.

```
>>> start_time = datetime.datetime.now()
>>> finish_time = datetime.datetime.now()
>>> (finish_time-start_time).total_seconds() * 1000
14567.464
>>>
```
Make that:

```
(delta.seconds * 1000000 + delta.microseconds)/1000.0
```