The instant and the wall time
Every event in the universe of computers happens at exactly one instant. A wall
time like “14:30” is what a clock in one particular place displays at that instant. The
same instant reads 09:00 in London, 10:00 in Paris, 17:00
in Tokyo and 08:00 in New York (on a winter day). None of those is “the real time”;
they are all views of the same thing.
UTC (Coordinated Universal Time) is the reference every other view is defined
against. It has no daylight saving and no offset: UTC+00:00 all year. Servers, logs,
APIs and aviation standardize on it precisely because it never shifts.
Offsets are not time zones
An offset like UTC−05:00 is a relationship between one zone and UTC at one moment.
A time zone is a full rulebook: its history, its daylight-saving schedule, and the exceptions a
government has legislated over the decades. New York is UTC−05:00 in winter and
UTC−04:00 in summer, so writing “New York is UTC−05:00” is only ever half true.
This is why serious systems store the IANA zone identifier
(America/New_York) rather than an offset. The identifier lets any computer resolve the
correct offset for any date, including dates decades in the past or future. Several
regions share one identifier (Boston and New York are both America/New_York); their
clocks agree by definition.
How to write a time that cannot be misread
- Store instants: as a Unix timestamp or an ISO 8601 string with an offset (
2026-10-28T14:30:00Z). - Display in the viewer’s zone: resolved at display time, because offsets depend on the date.
- Never store a naked local time:
2026-10-28T14:30:00without an offset identifies no unique instant; one system will read it as UTC, another as local.
ISO 8601 makes the intent explicit: Z means UTC, and +05:30 means “local
time 5 h 30 m ahead of UTC”. The ISO 8601 converter
parses and builds such strings and shows what offset-less input would mean under each
interpretation.
“GMT” and other near-synonyms
For everyday purposes UTC and GMT show the same wall time. Technically GMT is a time zone (the UK’s civil time in winter), while UTC is the atomic-clock standard that the world’s offsets are measured against. Leap seconds occasionally make them differ by a second in theory; civil timekeeping keeps them aligned.
Daylight saving is where it bites
Offsets move twice a year in about a third of the world’s zones, and not on the same dates. A meeting that is “09:00 New York / 14:00 London” in October may be “09:00 / 13:00” in November, because the US and Europe change clocks on different Sundays. Multiply that by participants in India (no DST, +05:30 always) and Australia (DST in their summer, opposite seasons) and hand-maintained offsets fail fast.
That is the case the Meeting Time Planner and Time Zone Converter solve: they resolve every participant’s offset for the exact date in question and flag day rollovers, instead of trusting a fixed offset.