Construct a DATE_RANGE string
date_range(start_date = date_calendar(), end_date = date_calendar())
A DATE_CALENDAR() object giving the start of the range. If only the start date is provided, a string of the form "AFT date" will be returned.
A DATE_CALENDAR() object giving the end of the range. If only the end date is provided, a string of the form "BEF date" will be returned.
A DATE_RANGE string.