Sydney is 15 hours ahead of Chicago. Put the other way round: Chicago is 15 hours behind Sydney.
Chicago is UTC-5 in summer and UTC-6 in winter; Sydney is UTC+10 and UTC+11.
A nine-to-five in Chicago and a nine-to-five in Sydney 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.
Chicago changes its clocks for summer and Sydney does not, so there is no single answer: the gap is 15 hours for the northern summer and 17 hours for the rest of the year.
| Chicago | Sydney |
|---|---|
| 08:00 | 23:00 |
| 09:00 | 00:00 next day |
| 10:00 | 01:00 next day |
| 11:00 | 02:00 next day |
| 12:00 | 03:00 next day |
| 13:00 | 04:00 next day |
| 14:00 | 05:00 next day |
| 15:00 | 06:00 next day |
| 16:00 | 07:00 next day |
| 17:00 | 08:00 next day |
| 18:00 | 09:00 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.