Why the same zone can show a different offset on different dates

A UTC offset isn't a fixed property of a time zone — it's a property of a specific moment in that zone, because most populated time zones observe daylight saving time for part of the year. America/New_York is UTC-5 in January and UTC-4 in July; picking a single "offset" for a zone without specifying which date you mean is technically an incomplete question. This tool always computes the offset for the exact moment selected, using the zone's real DST rules via the browser's Intl API, which is why comparing the same set of zones across two different dates in the year can legitimately produce different offset numbers even though the zone list didn't change.

Why this uses IANA zone names instead of abbreviations

Time zone abbreviations like EST, CST, or IST are ambiguous — CST alone could mean US Central Standard Time, China Standard Time, or Cuba Standard Time depending on context, and there's no registry that makes abbreviations unique globally. IANA zone identifiers (America/New_York, Asia/Kolkata) are unambiguous by design: each one names an actual region with a specific, continuously maintained history of UTC offset and DST rule changes going back for decades. That's also what lets this tool stay correct as DST rules change in the future — the browser's IANA database gets updated independently of this site, so a zone's rules here always match whatever the browser itself currently knows to be true.