What is the March 15th deadline for S-Corporations and Partnerships?

(disclaimer: this is not legal advice consult with a licensed professional)

Summary: No one forgets April 15th in the USA, but a lot of new businesses have not been educated or even know about March 15th. This is a reminder that if you run a business in the United States and are designated an S Corp Form 1120S or an entity Partnership or LLC with Form 1065 then you must file a return March 15th or file an extension.  The IRS will not be forgiving and there is no need to get hit with late penalties.