Why the date matters
A time zone is a set of rules, not a number. New York is five hours behind UTC in January and four hours behind in July, and the switch dates differ from Europe’s by up to three weeks. A converter that only knows “EST = UTC−5” is wrong for a large part of the year.
This calculator converts a dated local time using the named IANA zone you choose, so the offset is the one in force on that date. Each result card shows the offset, whether the place is on daylight or standard time, and a “+1 day” badge when the date changes.
Named zones and fixed offsets are different things
Asia/Kolkata and UTC+05:30 give the same answer today, but they are not the same: a named zone can change its rules (India used other offsets before 1945), while a fixed offset never moves. Cards are labelled “Named zone” or “Fixed offset”, and if both appear with matching offsets the page says why they still differ.
Pick a fixed offset only when the source literally says “+05:30”. For a city, choose its zone so daylight saving and future rule changes are handled.
Times that do not exist or happen twice
On the spring-forward night, local times in the skipped hour never appear on clocks. On the fall-back night, one hour repeats. When you enter such a time the calculator stops and asks: move forward, move back, use the first or second occurrence, or don’t convert. Your choice is included in copied text and exports.
More tools
Plan across working hours with the meeting planner, check when clocks change with the DST calculator, preview a recurring meeting through a DST change, convert epoch timestamps, and work out flight durations from local departure and arrival times.