The SEC has updated their rules relating to 13D filings to clarify that "for the purposes of calculating the 10-day time period, the first calendar day after the trade date counts as day number one."
Schedule 13D is a SEC filing that must be submitted to the US Securities and Exchange Commission within 10 days, by anyone who acquires beneficial ownership of 5% or more of any class of publicly-traded securities in a public company. 13D filings are of particular interest to corporations and to other investors since the filer must disclose the "purpose" of their investment. e.g. intention to obtain a controlling interest, demand for board representation, or simply because they believe the security is undervalued.
Schulte Roth & Zabel issued a client alert on this which can be downloaded here.