In mm/dd/yy, mm/01/yy is always the first day of the month.
Would make the date the first of the month.
If you ever need the last
day of the week, month, quarter or year, there are DFSORT functions for those. There are other DFSORT date functions, and it is worth familiarising yourself with them if you are doing date processing. There are further date manipulations in the Smart DFSORT Tricks book, available on the web.