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)
More straightforward:
```
try:
    datetime.strptime(processed_date, format_string)
except ValueError:
    return False
return True