"They aren't written off until they are paid (2019),"

The employee share of taxes are part of their gross pay. The date you pay the employees is the date you incur the Total Expense. The Banking (payment to State and Feds) is not the date of Expense; it's the date of Banking.

The same is true of the employer's share: it is Debit Expense and Credit Liability, owed for the pay date. Not the Banking date.

The fact that you "wrote it off" as 2018 means you should already have the liability entered; otherwise, you cannot have the Gross Payroll entered. The math is:

Gross Expense is the Debit.

Credits go to bank (for takehome), tax liability, and perhaps the employer's expense or asset account as reimbursement, such as Health Insurance premiums or employee draw/advance repayments.

Then, later, when you reach the tax payment due date, you pay out Liability; it's not expense at this point.