Moment.js is a popular JavaScript library used for parsing, validating, manipulating, and formatting dates and times. It simplifies working with dates by providing an easy-to-use API, allowing developers to perform complex date operations without dealing with the intricacies of the native JavaScript Date object. The library supports various date formats and time zones, making it versatile for different applications. Although it has been widely used, the developers have announced that it is now in maintenance mode, encouraging users to consider alternatives like date-fns or Luxon for new projects.