The Post Date on the Payroll Check Calculation Summary tab is used to determine which tax table formulas to use. If the date is set to last year, then taxes for last year are used. If the date is set to this year, then the current tax tables are used. Sometimes there are changes in mid-year, in which case the same principle applies; if the Post Date is after the date of the mid-year changes, the new tables are used.
Note: If you change the Post Date after the withholdings have been calculated, you’ll get a warning that they may need to be calculated again.