When you need to work with dates in JavaScript, the built-in Date object often feels clunky, especially for formatting and calculations. To address these challenges, modern libraries like date-fns have become popular. date-fns stands out for its functional approach—each operation is a pure function that takes a date (or dates) and returns a new one, without mutating the original. This style makes your code more predictable and easier to test. date-fns offers a wide range of utilities for formatting, parsing, adding, subtracting, and comparing dates, all in a modular package.

While both date-fns and Day.js offer modern solutions for date handling, they suit different needs. date-fns excels with its functional, modular design—each function does one thing and works with plain JavaScript dates, making it easy to pick only what you need and keep bundles small. This approach is ideal for projects where tree-shaking and functional programming are priorities. In contrast, Day.js provides a more object-oriented, chainable API, closely mimicking Moment.js, and is great for concise, readable code when you prefer method chaining. If you want flexible, composable utilities and minimal overhead, date-fns is often the better choice; if you prefer a Moment-like experience with a compact footprint, Day.js may be more comfortable.