Getting the number of days in a month enables accurate calendar calculations, date validation, and scheduling functionality in JavaScript applications. As the creator of CoreUI, a widely used open-source UI library, I’ve implemented month day calculations in numerous date picker and calendar components for enterprise applications. From my expertise, the most effective approach is using Date constructor with day 0 to get the last day of the previous month. This method automatically handles leap years and varying month lengths without complex conditional logic or lookup tables.