What is the deadline to file Form 941-X?

The due date for filing Form 941-X is dependent upon when you discover an error in your 941 Form.
Underreported Tax
To correct underreported tax, you must file Form 941-X by the due date of the quarter which you discovered the error. You must also pay the owed amount of taxes by the time you file.

You may correct underreported taxes on a previously filed Form 941 if you file Form 941-X within 3 years of the date the Form 941 was filed.
Overreported Tax—Adjustment Process
If you want to use the overpaid taxes as a credit to Form 941, simply file an adjusted return on Form 941-X soon after you discover the error but no later than 90 days before the period of limitations expires on the credit or refund for Form 941.

Generally, you may correct overreported taxes on a previously filed Form 941 if you file Form 941-X within 3 years of the date the original Form 941 was filed or 2 years from the date you paid the tax reported on Form 941, whichever is later.