I had a lot of help with this from Lars Appel, Jeff Vance, and Tim Ericson,
so they deserve the credit for me being able to put this together. But
except for leap months,
IF HPDATE=![WORD("31 28 31 30 31 30 31 31 30 31 30 31",,HPMONTH)]
will do it, and for catching leap day,
IF !HPDATE = 29 AND !HPMONTH = 2 AND !HPYEAR MOD 4 = 0
will fail on the first element except for 28 days of the year, so either
test, date or month, can go first. Of course, if the first two elements are
true, there's not much point in testing the third. There is more than one
way to put these two together to efficiently test both, depending on what
you want to do if it is the last day of the month.