A self-contained plugin that adds timezone support to dayjs. It implements conversion between arbitrary timezones using the Intl API, handles DST edge cases, and exposes methods like `tz()` and `utcOffset()`. The plugin is completely decoupled from the core—it only uses the dayjs instance and prototype methods—making it easily extractable for other projects that need timezone-aware date handling without pulling in a heavy date library. It demonstrates sophisticated handling of IANA timezone names and offset calculations.

Implements a `Duration` class as a plugin that adds duration arithmetic and formatting capabilities. It provides methods for adding, subtracting, comparing, and converting durations, as well as flexible formatting similar to Moment.js. The plugin is designed to work independently and can be reused in any date-related project that needs duration support. It showcases a clean, functional approach to handling time intervals without modifying the core API.

A collection of pure utility functions used across the core and plugins, including `padStart`, `padZoneStr`, `monthDiff`, `absFloor`, `prettyUnit`, and `isUndefined`. These are atomic, dependency-free helpers that solve common date math problems (diffing months, floor rounding, unit normalization). They are trivially reusable in any JavaScript project dealing with date manipulation and demonstrate a concise, functional style.