Why time zones are hard
A zone is not an offset. Europe/Belgrade is UTC+1 in winter and UTC+2 in summer, and the dates of the switch differ from America/New_York's, so the difference between the two cities is 6 hours most of the year, 5 hours for two weeks in March and 7 hours for a week in November. Some zones have changed rules repeatedly (Mexico abolished DST in 2022, Egypt reintroduced it in 2023), which is why software must use the IANA tz database with its history rather than a fixed offset table. This page uses the browser's copy of that database through Intl.DateTimeFormat.
Reading the results
Each row shows the local wall-clock time, the UTC offset in force on that date, the zone's abbreviation (CEST, EDT, JST…) and whether daylight saving is active. The ±1 day marker tells you when the local date differs from the base date, which is what makes “Tuesday 09:00” in Sydney a Monday-evening call for Europe.
Scheduling across zones
- Pick the time in the zone of the person who moves least often, or in UTC, and state the zone in the invitation.
- Use the grid: a column that is green or amber in every row is a workable slot. Three zones spanning Europe, the US west coast and East Asia usually have none; then someone takes an early or late slot in rotation.
- Avoid the two weeks each spring and autumn when Europe and North America are out of step; recurring meetings shift by an hour for one side.
- Store timestamps in UTC; convert at display time. Store future appointments as local time plus zone, because the rules may change before the date arrives.
Questions
What is the difference between CET and Europe/Belgrade?
CET is an abbreviation for a fixed offset (UTC+1), ambiguous in summer when the same places use CEST. Europe/Belgrade is an IANA zone with the full rule history. Always use the zone name in software.
Why is the time difference not constant?
Because the two zones switch daylight saving on different dates, or one does not switch at all. The planner recomputes the offset for the exact date you enter.
Which time should I put in an invitation?
The time in the recipient’s zone, with the zone name, or a UTC time. Calendar invitations carry the zone, so the recipient’s calendar converts automatically; plain-text messages do not.
Does the converter handle historical dates?
Yes, as far as the browser’s tz data goes, which covers rule changes back to the 1970s and often earlier. Future dates use the rules currently published; governments change them with little notice.