When you work with dates and times in JavaScript, the first tool you encounter is the built-in Date object. While Date provides basic methods for creating, manipulating, and displaying dates, it has many well-known limitations that can cause problems in real-world projects. Developers often struggle with inconsistent parsing of date strings, confusing zero-based months, and unreliable handling of time zones and daylight saving time. These issues can lead to bugs that are difficult to reproduce, especially when your code runs in different browsers or environments. For example, parsing the same date string may yield different results in Chrome and Firefox, or on different operating systems. Formatting dates for display can also be verbose and error-prone, making even simple tasks unnecessarily complex.

To address these challenges, Day.js was created as a modern alternative for date handling in JavaScript. Day.js is designed to be as lightweight as possible—just a few kilobytes in size—so it loads quickly and adds minimal overhead to your web projects. Its API is modeled after the popular Moment.js library, but Day.js emphasizes simplicity, immutability, and chainable methods for a better developer experience. The core philosophy of Day.js is to provide a consistent, predictable, and easy-to-use interface for all your date and time needs, without the bloat of larger libraries or the quirks of the native Date object. This makes it especially appealing for modern web development, where performance and reliability are critical.