I understand form 990 is due on the 15th day of the 5th month following the end of the organization's taxable year. For organizations on a calendar year, the Form 990 is due on May 15th of the following year. I have a new client bringing in their filed tax year 2010 form 990 for which they believe needs amending. I did ask if the non-profit has a calendar tax or not and they didn't know right off the bat. Being only Jan of 2014, I would think they still have time to amend their 2010 tax return.