Understanding the 360-Day Year In commercial transactions it is traditional to use a 360-day year. The historic rationale for this procedure was ease of calculation which made sense before the com puter and calculator age Why does this practice continue when most small calculators and desktop computers can present complex interest calculations in a few seconds? c7-18C7 - 18 In commercial transactions it is traditional to use a 360-day year. The historic rationale for this procedure was ease of calculation which made sense before the computer and calculator age. Why does this practice continue when most small calculators and desktop computers can present complex interest calculations in a few seconds? Understanding the 360-Day Year