During the first week of January, the IRS released updated tax withholding tables for 2013 based on the income tax changes in the new law. (IRS Notice 1036) New withholding tables also reflect the end of the payroll tax holiday for Social Security tax.
Alert: Employers should start using the revised withholding tables right away, but in any event no later than Feb. 15, 2013.
For any Social Security tax that is under-withheld before that date, make the appropriate adjustments as soon as possible, but no later than March 31, 2013.
Tip: For more details, visit the IRS website.