Use Form 1120X to:
Correct a Form 1120 or 1120-A as originally filed, or as later adjusted by an amended return, a claim for
refund, or an examination, or
Make certain elections after the prescribed deadline
(see Regulations sections 301.9100-1 through 3).
File Form 1120X only after the corporation has filed its
original return. Generally, Form 1120X must be filed
within 3 years after the date the corporation filed its
original return or within 2 years after the date the
corporation paid the tax (if filing a claim for a refund),
whichever is later. A return filed before the due date is
considered filed on the due date. A Form 1120X based on an NOL carryback, a capital loss carryback, or general business credit carryback generally must be filed within 3 years after the due date (including extensions) of the return for the tax year of the NOL, capital loss, or unused credit. A Form 1120X based on a bad debt or worthless security must be filed within 7 years after the due date of the return for the tax year in which the debt or security became worthless. See section 6511 for more details and other special rules.
It often takes 3 to 4 months to process Form 1120X.