Day.js is a lightweight JavaScript date/time library (approximately 2 kB gzipped) designed as a drop-in alternative to Moment.js. It provides immutable, chainable parsing, validation, manipulation, formatting, and comparison APIs, along with extensive internationalization support via over 100 locale files. The library is built as a modular core with a plugin system that extends functionality without bloating the base bundle. It runs in both browsers and Node.js, includes TypeScript definitions, and has a comprehensive test suite that ensures behavioral parity with Moment.js. This is a library, not an application or data pipeline—there are no server components, CLI tools, or data analysis artifacts. The architecture is intentionally small and focused, with a constant-driven unit system and a clean separation between core, plugins, and locales.