Zurich is 8 hours ahead of Denver. Put the other way round: Denver is 8 hours behind Zurich.
Denver is UTC-6 in summer and UTC-7 in winter; Zurich is UTC+2 and UTC+1.
A nine-to-five in Denver and a nine-to-five in Zurich do not overlap at all. Somebody has to move: the nearest workable slots are early morning in one city or late evening in the other, which is the kind of thing the planner is for.
Both cities move their clocks for summer, and by the same hour, so the gap above holds for almost the whole year. It slips by an hour for the days between the two countries' switch dates, which do not line up exactly.
| Denver | Zurich |
|---|---|
| 08:00 | 16:00 |
| 09:00 | 17:00 |
| 10:00 | 18:00 |
| 11:00 | 19:00 |
| 12:00 | 20:00 |
| 13:00 | 21:00 |
| 14:00 | 22:00 |
| 15:00 | 23:00 |
| 16:00 | 00:00 next day |
| 17:00 | 01:00 next day |
| 18:00 | 02:00 next day |
This page covers one pair. TimeGap takes a list: add every city you care about, drag one marker, and every row reads the same instant on its own clock. It marks the hours you would actually take a call, finds the window where the most people are reachable, flags public holidays, and puts the time you pick straight into Outlook or Google Calendar.