"My dog is less than a year, 4 months old"
versus
"My dog is zero years, plus 4 months old"
It suggests then I should have used :years and :keys and made them both optional in order to make it consistent for a human reader. Or leave it as-is and use (= years 0) since that reflects my input example where year is always input, even if 0.
It all still works the same. I'm just trying to make it as lisperifically correct as it can be.