Clock Web App is an Open Source project that brings a feature rich Clock app to any system for completely free. It is developed for Web Based Operating Systems and is powerful software that is easy to use and easy to develop as it has a clean code base. Clone the repo, serve the public/ subdirectory, and that's it. No build process.

TimeZoneDB provides free time zone database for cities of the world. The database is licensed under Creative Commons Attribution 3.0 License. It contains countries name, time zones, abbreviation, GMT offset, and Daylight Saving Time (DST). The data is available in CSV and SQL format. You can download and implement into your projects for free.

API access also available to query time zone information. You can get time zone data by latitude & longitude, or zone name. You need to register a API key to access the API.

An on-line web application that shows you what time it is in every other time zone around the world relative to the current time. Handy if you're trying to figure out if you should ping someone at a particular hour.