>A user enters a day in a spinner. They then choose a month from a combobox. They do not enter a year. Is there an
>easy way to tell if the month/day combination is valid?

You could do:

IF EMPTY(CTOD(STR(month,2)+"/"+STR(day,2)"))
  * invalid
ELSE
  * valid
ENDIF

You only need the month and day to make a valid date (assumes current year).
You may have to special checking to handle Feb. 29.

Note that this is SET DATE format specific.