So, your program needs to calculate what date that is
Yes it does, Qnd that is harder for the programmer and easier for the user. Calendar wudguts suck for:
> last week of august next year.
Twenty years ago parsing user input and calculating a date was resource constrained and ambiguity was expensive because the query was hitting Sabre on some mainframe. Today we've got gigahertz and gigabytes in our pockets and we're wired up at megabit speeds to caches and CDN's and search tools with autocompletion (Windows Intellisense)