from datetime import datetime


def datetime_string_to_naive(datetime_string):
    """
    This function converts ISO 8601 time string without timezone info to a naive datetime object.
    Main usage is for date of birth and other places where timezon is not required.
    """
    try:
        dt = datetime.strptime(datetime_string, '%Y-%m-%dT%H:%M:%S')
        return dt
    except ValueError:
        return None