Regarding LLCs, the due date depends on how the businesses are taxed and also their fiscal year. If the business is taxed as a partnership or an S-Corp and their fiscal year is January 1 to December 31, then you will need to file an extension for the business by March 17 of this year. Businesses can also be an LLC and be taxed as a sole proprietor; in that case the due date this year is April 15.
You are correct that C Corps who follow the calendar year, have a due date of April 15.
For more information about business due dates and how to file business extensions, see the following TurboTax help articles:
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