Check that inputs are valid dates

I have a method in a class that accepts either a datetime or string parameter. It is shown in the last code block (I didn't include the class). A little background is that I am defining a wrapper around an API, and I want the end user to be able to pass either a string or a date but convert that into the appropriate string for the API query.

I wrote a decorator that validates the dates, and converts the datetime to the appropriate string format for the subsequent call to the API. The decorator I wrote is defined outside of the class as a function.

1. Should I include validate_date_helper in the scope of the wrapper?
2. Is defining these functions outside of the class pythonic and is this a valid approach for what I am trying to accomplish?
3. Are there other ways to accomplish validations like this?

from datetime import datetime as dt
from typing import Union

def validate_date_helper(date):
     """
     validates a start date or updated after date.
     end
     """
     utc_format = "%Y-%m-%dT%H:%M:%S"
     if isinstance(date, dt):
         date = date.strftime(utc_format)
     elif isinstance(date, str):
         utc_format_date = date + "T00:00:00"
         dt.strptime(utc_format_date, utc_format)
     else:
         raise ValueError(f"{date} must be datetime object or string")
     return date


def validate_dates(func):
     def func_wrapper(self, *args, **kwargs):
         """
         validates dates for the between function
         """
         new_args = []
         new_kwargs = {}
         for arg in args:
             arg = validate_date_helper(arg)
             new_args.append(arg)
         for key, val in kwargs.items():
             if val is not None:
                 val = validate_date_helper(val)
             new_kwargs[key] = val
         res = func(self, *new_args, **new_kwargs)
         return res
       return func_wrapper



@validate_dates
def between(self, start: Union[dt, str], end=None):         
    """
    sets startime and endtime for API query
    """
    self.query["starttime"] = start # self.query is an empty dictionary at class instantiation
    if end is not None:
        self.query["endtime"] = end  
    return self