William - I had a hunch who I should trust, but points is points and you can't be too careful ....

I followed up with customer service and received back the following. It answers my particular question, but the full explanation seems odd and I'm not sure it's even a question I've seen asked before. I post it only for commentary and reference (and perhaps a bit of amusement, sorry), as my question has been answered.

"yes, March 31st would count as long as that stay is only for one night. If you were to stay longer, then the stay would technically include nights in April and disqualify you for the promotion."

I think it further confirms who I should trust going forward