def num_periods(period_duration, last_fixed_date, current_date):
    """Return the number of periods in a date range.

    Arguments:
        period_duration - the timedelta of a period.
        last_fixed_date - the date that the daystate was last specified.
        current_date - the date in question.
    """
    return (current_date - last_fixed_date) / period_duration