I can see value in Manager supporting some date arithmetic functions in values displayed to the customer / employee. In business I routinely set clear requirements but only demand penalty payments when the written requirements are clearly and unambiguously exceeded. Getting too trigger happy increased the risk I, rather than my customer, is at fault and cost me more in loss of goodwill rather than I gain in regular bill payment. Calculating pay period dates is another potential application of date arithmetic
I suspect the solution would be part of Manger supporting calculated values during data entry, which has already been suggested to payslip taxation, depreciation schedules and other applications.