When working with Day.js to access or change parts of a date, you use getter and setter methods like year(), month(), date(), hour(), minute(), and second(). Each getter returns the current value for that part of the date. To set a component, pass a value to the method, which returns a new Day.js object with the updated value.

A key detail to remember is that months in Day.js are zero-based: January is 0, February is 1, and December is 11. This matches the JavaScript Date object, so always subtract one from the human-readable month number.

Day.js is immutable. This means that any setter method like year(2025) or month(0) does not change the original date object—it creates and returns a new Day.js instance with the modification. This prevents accidental changes and makes your code more predictable.

For best results when manipulating date components:

• Always assign the result of a setter to a new variable;
• Be careful with zero-based months to avoid off-by-one errors;
• Chain setters if you need to update multiple components in sequence;
• Use format() to display the result in a human-friendly way.