Convert Python Datetime To Readable String
```python
def get_duration(start_time, end_time, return_format=None):
# start_time: datetime object
# end_time: datetime object
# See below for acceptable return_formats
# Duration in seconds
duration = int((end_time-start_time))
# If a return_format was provided
if return_format:
if return_format == 's':
# Seconds, do nothing, as it's already in seconds
duration = f'{int(duration)} seconds'
elif return_format == 'm':
# Whole minutes
duration = f'{int(duration / 60)} minutes'
elif return_format == 'ms':
# Minutes and seconds
mins = int(duration / 60)
duration = f'{mins}m {duration-(mins*60)}s'
return duration
```