1
Date arithmetic is not just day counting
Date arithmetic seems simple until you cross month boundaries, leap years, and irregular calendar lengths. Adding 30 days is not always the same as adding one calendar month, and adding one year to February 29 requires a rule for non-leap years. This is why date calculators must distinguish between fixed-duration movement and calendar-based movement.
In billing, contracts, subscriptions, and project planning, users often mean “the same day next month,” not “30 days later.” That subtle difference can materially change deadlines and renewal timing.