What do the three shades in the grid mean?

The legend under the heatmap names them Work hours, Fringe (early/late) and Asleep. Work cells carry the site's teal tint, fringe an amber one, asleep plain grey. An hour counts as fringe if it falls within 90 minutes of either edge of the working window you set, or simply if it lands between 07:00 and 23:00 local, so asleep only turns up for hours roughly between 23:00 and 07:00. Switch to Timeline and the same three states are drawn green, amber and grey.

Where is the guaranteed overlap shown?

Switch to the Timeline view. An Overlap bar appears above the city rows, green on every hour where all cities are inside their working hours. If there is no such hour, the bar is not drawn at all, which is your signal that someone will have to take the call outside their window.

Can I export the meeting to my calendar?

Yes. Three buttons sit under the local-times list: Download .ics, Google Calendar and Outlook. The .ics file and the Google link both carry a description listing every city's local time for the slot. The Outlook deep link carries only the title, start and end.

How is the best slot chosen?

Every hour is scored by counting cities in their working hours, weighted three times, plus one point for each city in fringe hours. The highest score wins, and the earliest hour wins a tie. It is a count of people, not a judgement about seniority or fairness across weeks.

Does my meeting data get uploaded anywhere?

The planner runs entirely in your browser and stores your city list in local storage on your device. The Google Calendar and Outlook buttons are ordinary links that open those services with the event details in the URL, so those two do send the title and times to the vendor you clicked.

Plan a Meeting Across Time Zones

Pick a meeting hour that suits every city: read the overlap heatmap, set each person's working hours, then export the slot to .ics, Google Calendar or Outlook.