Summarize data

Please, check my code!
I forgot to include some kind of readme.st. To test custom command:
python manage.py summarize [date YYYY-MM-DD] [--nodeletion]
If you pass no parameters it summarizes current date minus 15 days (single day and no a 15-days period)
I see,
I considered this function as a predicate. So I was expecting it to return only Boolean. Exception is also a good idea. But I think if it raises an exception, it should not return True in success case. Because, we will only be interested in side effect of the function. Something like this maybe:

```
try:
    datetime.strptime(string_date, format_string)
except ValueError:
    raise CommandError('Date should have the right format YYYY-MM-DD')
```