A payroll tax cut that has been in effect since 2011 will extend through the end of February. The Temporary Payroll Tax Cut Continuation Act of 2011 temporarily extends the two percentage point payroll tax cut for employees, reducing their Social Security tax withholding rate from 6.2 percent to 4.2 percent of wages.  According to the Internal Revenue Service, the reduction will have no effect on employees' future Social Security benefits.

Employers are advised to implement the new payroll tax cut as soon as possible, but no later than Jan. 31. For any Social Security tax over-withheld during January, employers should make an offsetting adjustment in workers' pay no later than March 31.

Under the terms negotiated by Congress, the law also includes a new "recapture" provision, which applies only to those employees who receive more than $18,350 in wages during the two-month period (the Social Security wage base for 2012 is $110,100, and $18,350 represents two months of the full-year  amount). This provision imposes an additional income tax on these higher-income employees in an amount equal to 2 percent of the amount of wages they receive during the two-month period in excess of $18,350 (and not greater than $110,100).