Norman Alexander
Penultimate Amazing
It's best not to leave calculations in the hands of the end users. Make them tell you what they think they want, gently correct them into wanting the right thing, then return results that nobody can prove aren't precisely what they think you meant when you told them what they're supposed to mean. If there are any questions explain the mechanics of the logic very thoroughly until they give up and accept what you already gave them.
Especially given their obstinate proclivity for getting data entry wrong, PARTICULARLY DATES! Every programmer will tell you that there is yet to be a 100% successful date entry routine that a user cannot break.