Your clients are probably aware that the Internal Revenue Service and the Treasury Department have extended the federal income tax filing and payment deadline for the 2020 tax year from April 15, 2021 to May 17, 2021. Be aware, however, that clients might overlook the fact that this extension applies only to individual taxpayers. 

And although the May 17, 2021 deadline does apply to individuals who pay self-employment tax, it does not apply to estimated tax payments (still due on April 15) or withholding. The extension also does not apply to nonprofits and business entities, so they still need to file Forms 1120 and 990 by the April 15 deadline. In addition, individuals still must comply with state filing deadlines, to which an extension may or may not apply.