first_last.RdWithin a time unit (boundary), find ceiling (upper, max) and floor (lower, minimum) date. The default time unit (days) return the date back.
round_date picks the closest date from ceiling and floor.
<unit>_first and <unit>_last gives the first and last day within the time unit.
Use the index argument to pick the time unit level (example use 6 to pick up june in month_first).