Don’t forget that the 2018 tax tables were not released until January 11, 2018, and employers were not required to put them in place until February 15, 2018. This means that for perhaps as much as 6 or 7 weeks in 2018, you may have been using 2017 tax tables for your 2018 payrolls.
What You Need To Know
If you attempt to prove the 2018 W2 Box 2 figure for federal income tax withheld, you will not be able to rely on the 2018 tax tables alone.
You must know exactly how many payrolls you ran using the 2017 tax tables and exactly when you began applying the 2018 tax tables. Remember that the dates will be different if you have multiple pay frequencies, like a weekly payroll, and a biweekly payroll, and a semimonthly payroll.
Only after you have determined the dates where you applied 2017 tax tables and the dates where you applied 2018 tax tables will you be able to split your calculations between 2017 and 2018 tax formulas so that you can prove the amounts for federal income tax withheld on 2018 Forms W2.