Retrocalculations are what payroll engines do when a prior period needs to be restated. Triggers are not just backdated salary changes. They include hire or termination dates entered wrong and corrected later, absence adjustments, benefits election changes, retroactive cost-center reassignments, and any other prior-period change. The engine recomputes affected pay periods, generates retro deltas, recalculates statutory deductions, and creates new retro records alongside the originals. Generic tools see one source change. The target generates dozens of secondary records. Reading those records back requires payroll-specific knowledge that iPaaS recipes and unified APIs do not have.
