Day.js is designed with a minimal core, making it lightweight and fast. To add advanced features, you can use plugins. Plugins let you opt into only the functionality you need, keeping your project efficient.

Popular plugins include relativeTime for displaying phrases like "2 hours ago," duration for working with time spans, utc for universal time handling, and timezone for managing time zones.

To use a plugin:

• Load it after Day.js;
• Call dayjs.extend(plugin);
• Choose plugins based on your requirements.

Use plugins when you need features not found in the core, but avoid adding unnecessary plugins to keep your bundle size small. If you need custom behavior, you can even write your own plugin to extend Day.js for your project.