old payroll tax liability

Code it to payroll tax expense. 

If it is a material item, you might want to talk to your tax preparer to see if an amended income tax return is necessary.

I agree with IRMN. If it is a material amount it should be posted to an equity account as a prior period adjustment. If the prior year is still open then it should be corrected by increasing the expense account and the payroll tax liability account. If it effects W-2 forms you may need to issue corrected W-2's. If it is not particularly material you could just expense it to a current payroll tax expense account or create an additional expense account for "past payroll taxes" in order to segregate it from the current payroll tax expenses. Amending is really the only totally correct way but it may not be cost effective to amend for a small amount. You may also want to consult with your tax preparer and get their preference as well.