def format_time_filter(start, stop, field):
    """Formats a time filter for a given endpoint type.
    Args:
        start (str): An ISO date string, e.g. 2020-04-20.
        stop (str): An ISO date string, e.g. 2021-04-20.
        field (str): The time field to filter by.
    """
    start_date = start.split('-')
    end_date = stop.split('-')
    y1, m1, d1 = start_date[0], start_date[1], start_date[2]
    y2, m2, d2 = end_date[0], end_date[1], end_date[2]
    return f'?f_{field}1={m1}%2F{d1}%2F{y1}&f_{field}2={m2}%2F{d2}%2F{y2}'