Create a function to validade possible date formats in serializer

[dunossauro]

Please, don't think only about API request to do this issue. 'Cause the JSON can be directly sent by CLI.

IMO we need only a simple formats, like:

1. ISO -> formal python datetime
2. 'MM/AAAA' -> Brazilian month/year
3. 'AAAA/MM' -> International date format.

Remember, in resume we don't need to specify days, minutes, hours, ...