When you form a C corporation, you establish your fiscal year from the beginning. Once this is set, you must follow it for the life of the corporation, unless you apply to the Internal Revenue Service for a change. The procedures for this are straightforward, but the consequences are not. In some cases you may have to pay a fee, and you may even be turned down for permission to make the change.
The Internal Revenue Service counts a C corporation's fiscal year as the one it established in the first year of operation. In order to change this, you must file an amended return with a request to change your fiscal year. You cannot merely file for an extension or apply for a new taxpayer identification number to change your tax year starting and ending dates.
You must fill out IRS form 1128 to request a change in tax year for a C corporation. The IRS requires you ask for a "Ruling Request" on this form as part of its criteria to approve a fiscal year change. The Form is relatively straightforward to fill out but it is helpful to read the form 1128 instruction document before you begin. Updated instructions are issued by the IRS each year.