If you need to correct W2 forms that have already been mailed or e-filed with the SSA, you must file a W-2C Correction Form, with a W-3C form to summarize corrections by a single employer. Buy W-2C Correction Forms >
There is no deadline, simply file the form as soon as you discover the error.
If you find an error before filing a W-2 Form with the Social Security Administration, you DO NOT need to file a W-2C form, even if you have already mailed the employee copy. Before mailing W2 Copy A forms to the SSA, simply check the “Void” box at the top of the incorrect W2, and create a new W-2 with the accurate information. Send Copy A to the Social Security Administration. For employee copies B, C and 2, write “CORRECTED” on new copies and mail them to the employee.