How to Use This Tool
Scheduling across timezones goes wrong for two reasons, and neither is arithmetic. The first is that people convert times in their head using a remembered offset, and offsets change twice a year on dates that differ by country. The second is that a converted time can be technically correct and still be 06:30 for somebody who did not want to say so. This tool solves both by projecting everyone's actual working window onto one shared axis and scoring every hour of the day.
Setting up the three participants
Each card takes a label, a timezone and a working window. Timezones are real IANA zones such as
Europe/London or Asia/Singapore, not fixed offsets — this matters, because a
zone carries its own daylight saving rules and its own transition dates. Choosing "UTC+1" would silently
break in late October; choosing Europe/London does not.
Set each person's window honestly. The default 09:00–17:00 is a fiction for most people. If someone genuinely will not take a call before 10:00, enter 10:00 — the entire value of the tool comes from the windows being real. You can also give one participant a wider window to represent the person who has volunteered to take the awkward slot.
Reading the grid
The table shows all 24 hours on your chosen date, with each participant's local time in their own column. Colour is scored per person: green means the slot sits inside their stated window, amber means it is within an hour of the edge, and red means it falls outside entirely. The right-hand column scores the hour overall. An hour is only offered as a best slot when it is green for everyone for the full meeting duration — which is why changing the duration from one hour to ninety minutes can eliminate a slot that looked fine.
The daylight saving warning
If the chosen date falls within two weeks of a DST transition in any of the three zones, a warning appears. This is the failure mode that quietly ruins recurring meetings: the United States, Europe and Australia all change on different dates, so for several weeks each year the gap between two cities is an hour different from what everyone has memorised. A meeting scheduled during that window will drift for half the attendees. If you see the warning, confirm the date rather than the offset.
Practical scheduling advice
- Rotate the pain. If no green slot exists for all three, someone must take an early or late call. Alternate who it is rather than defaulting to the most junior timezone.
- Anchor recurring meetings to a city, not an offset. "14:00 London" survives DST; "13:00 UTC" quietly moves relative to everyone's working day.
- Send invitations with the timezone named. Calendar software converts correctly, but the email describing the meeting usually does not.