Question: What is the deadline for submitting IRS Form 1040X to amend a federal income-tax return?

Tom Herman for the Wall St. Journal writes: Answer: The deadline typically is three years from the date you filed your original return. But this subject can be surprisingly tricky, and there are some exceptions to the general rules.
Generally, you have to file Form 1040X "within three years (including extensions) after the date you filed the original return" in order to get a credit or refund (or within two years of the date you paid the tax, whichever is later), according to the instructions in Form 1040X (available at www.irs.gov). 

"If you filed your original return early (for example, March 1 for a calendar year return), your return is considered filed on the due date (generally April 15)," the Internal Revenue Service says. "However, if you had an extension to file (for example, until Oct. 15) but you filed earlier and we received it July 1, your return is considered filed on July 1."
Unfortunately, you can't file an amended return electronically. You have to send it via old-fashioned snail mail.

The filing deadline can be different in certain cases. Among them: 

Bad debts or worthless securities. The time limit in these cases is "within seven years after the due date of the return for the tax year in which the debt or security became worthless," the IRS says. 

Mental or physical issues. The time limit can be "suspended" for "certain people who are physically or mentally unable to manage their financial affairs," the IRS says. IRS Publication 556 has details.
Carrybacks. You can file either IRS Form 1045 or Form 1040X to apply for a refund based on "the carryback of a net operating loss, an unused general business credit" or certain other items. See the instructions for Form 1045 for details on the deadline.