A Date Difference calculation returns the elapsed interval between two dates as a numeric value. Workday supports intervals such as years, months, days, hours, minutes, or seconds, depending on the calculation context. For this requirement, the calculation uses the worker's hire date and the applicable period start date and returns the completed number of years between them.
That numeric result can then drive service-based accrual logic. For example, the value may be evaluated directly in a conditional calculation or used as search criteria in a numeric lookup table to return different accrual rates for workers with fewer than five years, five through nine years, or ten or more years of service.
A Date Extract calculation returns only a component of one date, such as its year or month; subtracting extracted calendar years can produce an incorrect service result when the employment anniversary has not yet occurred. A Lookup calculation returns a configured value after criteria have already been calculated, while a Conditional calculation selects a result based on conditions. Neither independently calculates the elapsed years. The Date Difference calculation is therefore the correct foundational numeric calculation for determining service duration.
Reference: Workday Education - Workday Reporting: Calculated Fields, "Date Difference" ; Workday Education - Payroll for Administrators: The Calculation Engine