With guidance from the American Institute of Certified Public Accountants (AICPA), new legislation was approved changing filing deadlines for many common returns.
For example, deadlines for pass-thru entities - partnerships and S-Corps - are now due March 15, which is pushed up a month from previous due dates (but are still eligible for 6-month extensions to September 15). Returns for calendar year C Corps are now due April 15 - the same as those for individual and sole proprietor returns.
For a complete listing of the changes, visit this link from the AICPA website.
For a discussion as to the history and reasoning behind the change, visit this link from the Journal of Accountancy.