Why are wages reported as an expense when the work occurs, but the employees' tax records report them when they are paid?

The short answer is that the corporation's financial statements are prepared using the accrual method of accounting (generally accepted accounting principles), while the payroll records for the government uses the cash method of accounting (IRS rules).

To illustrate, let's assume that a new retailer (that uses the accrual method of accounting) begins operations on December 27. The employees working from December 27 through December 31 will receive their first paychecks on January 4. If the employees earn a total of $5,000 of wages during the five days ending on December 31, the retailer's general ledger and financial statements for the period ending December 31 must report this expense of $5,000 and a liability of $5,000 as of December 31. In accounting jargon, the retailer must accrue the expense and liability through an adjusting entry dated December 31.

However, the retailer's payroll department must file reports with the Internal Revenue Service (IRS) for its employees' wages according to the cash method. Therefore, the IRS reporting will be based on the pay dates. Since January 4 is the first time the employees can access their pay, the employees' wages earned during December 27-31 will be reported to the IRS as January wages.