We completely overhauled the checkout system’s date calculation and architecture by shifting away from fragile, hardcoded date jumps and manual DOM targeting. To meet the business rules for delivery options, we implemented an isWeekend utility and a day-by-day while loop that dynamically skips Saturdays and Sundays, ensuring shipping timelines stay perfectly accurate relative to weekends. Throughout the build, we ran into and fixed several tricky JavaScript edge cases, like catching case-sensitivity crashes where cartItem.Quantity broke data binding, declaring implicit globals to prevent local variables from leaking between layout generation loops, and adding safe fallback defaults for unassigned delivery option IDs. By stripping out tightly coupled element selectors from our quantity updates and converting the logic to a clean, true MVC structure where data mutations trigger a complete structural re-render (renderOrderSummary() and renderPaymentSummary()), the application now maintains flawless UI synchronization and data syncing completely on the fly.