Pick a wall-clock time in one zone, read it in another — daylight saving handled by the browser’s own zone database, not a lookup table from 2019.
Yes — conversions use your browser’s IANA time zone database (the same one your OS clock uses), so DST transitions, half-hour zones like India, and oddities like Lord Howe Island all come out right for the specific date you picked, not just “the usual offset”.
Zones are named by city (America/New_York, not “EST”) because abbreviations are ambiguous — there are three different CSTs. Your own zone is preselected as the starting point.
Entirely in your browser. Dates you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.