I would say that waiting until March 31 would require language similar to "at the closing bell on March 31, 2008." Using the word "by" in place of "at" should trigger this event at any point in time between when it starts and the end-date.

If we have a proposition that Yahoo! will accept Microsoft's offer by March15th, we would consider it a success if the acceptance is made on March 14th. If we have a proposition that Microsoft will release an interactive 3D technology ON February 29th, then releasing said technology on Feb 28th or March 1st would result in failure.

Prepositions matter. And they have meaning.