human
I wrote the below code

    def datetime_type(string):
        '    Validates    UTC    datetime.    Examples    of    accepted    forms:\n                                2017-12-31T01:11:59Z,2017-12-31T01:11Z    or    2017-12-31T01Z    or    2017-12-31    '
        accepted_date_formats = ['%Y-%m-%dT%H:%M:%SZ', '%Y-%m-%dT%H:%MZ', '%Y-%m-%dT%HZ', '%Y-%m-%d']
        for form in accepted_date_formats:
            try:
                if to_string:
                    return datetime.strptime(string, form).strftime(form)
                return datetime.strptime(string, form)
            except ValueError:
                continue
        raise ValueError("Input    '{}'    not    valid.    Valid    example:    2000-12-31T12:59:59Z".format(string))
    return datetime_type

What's a good function header?
assistant
#Here's a good function header.

def get_datetime_type(to_string):