Offsets, zones and why the distinction matters
An offset such as +01:00 describes the difference from UTC at one instant. A time zone such as Europe/London is a rule set covering that region's entire history of offsets, including daylight saving transitions and past legislative changes.
Confusing the two causes a specific and common class of bug. Storing an offset with a future event does not survive a daylight saving transition — an event stored as 09:00+01:00 for a date in November is actually 08:00 local time once the clocks change. Storing the zone identifier instead allows the correct local time to be computed whenever the rules say.
Zone rules change more often than people expect. Governments alter daylight saving arrangements, sometimes at short notice, and the IANA time zone database is updated several times a year to track it. Software that has not updated its copy will compute the wrong local time for affected regions.
Abbreviations are ambiguous
Time zone abbreviations should not be used in anything that must be unambiguous. CST is Central Standard Time in North America, China Standard Time and Cuba Standard Time; IST is Indian, Irish and Israel Standard Time. There is no registry making them unique, and several map to more than one offset.
There is a second layer of ambiguity: whether an abbreviation implies standard or daylight time. EST and EDT differ by an hour, and someone writing "EST" in June usually means Eastern Time, currently on daylight saving, which is EDT. Taking it literally schedules the meeting an hour wrong.
The unambiguous forms are a UTC offset, or an IANA identifier such as America/New_York. For invitations, stating the time in the recipient's zone and including UTC removes the guesswork entirely.
Scheduling across zones
Transitions do not happen simultaneously worldwide. The US and Europe change on different dates, and the gap — typically a few weeks in spring and autumn — shifts the difference between New York and London from five hours to four and back. Recurring meetings scheduled during one period silently move relative to each other during the gap, which is the most common cause of a standing call arriving an hour off twice a year.
The southern hemisphere is offset by a season, so Australia and much of South America move in the opposite direction from Europe and North America. Many countries observe no daylight saving at all, including most of Asia and Africa, and a few zones use offsets that are not whole hours — India at +05:30, Nepal at +05:45, and parts of Australia at +09:30 and +08:45.
For recurring meetings across regions, anchoring to one location's local time and letting others shift is usually clearer than anchoring to UTC, since at least one group has a stable time. Whichever you choose, say which — and for scheduled jobs rather than meetings, the cron parser covers the related pitfalls of running work across transitions.
Posting a time in Discord? Use the Discord time zone converter instead — it writes a tag that shows in each member's own local time, so nobody converts anything.