Moment.js is a popular open-source JavaScript library designed for parsing, validating, manipulating, and formatting dates and times. It provides a comprehensive and easy-to-use API for handling the often complex requirements of date and time operations in web development. Despite its widespread use, it is considered a legacy project since more efficient and modern alternatives like date-fns and Luxon are available. However, it remains a valuable tool for projects that benefit from its extensive functionality and support.