Dato is a replacement for the system menu bar clock. By default, it looks exactly the same, but when you click it, you get a small calendar, calendar events, the current time in various time zones, and more.
Browser Extension that replaces your New Tab with a minimalist app to help you track multiple timezones. To try it out on the web visit https://mrwolf.app No Tracking. 100% Free and Open Source. Source code available at https://github.com/beauallison/mrWolf/