Time zone abbreviations are convenient and quietly dangerous. IST means Indian Standard Time at UTC+05:30, Irish Standard Time at UTC+01:00 and Israel Standard Time at UTC+02:00. CST covers a North American zone, a Chinese one and a Cuban one. This table lists the common abbreviations, what they stand for, and where they collide.
Use them in prose, never in data
Abbreviations are fine in a sentence where context is obvious. They are a poor choice in a calendar invite, a database column or an API payload, because there is no authority that makes them unique. Record a UTC instant plus an IANA zone identifier instead — the reasoning is set out in time zones for developers.
Standard versus daylight forms
Most North American abbreviations come in pairs: EST in winter and EDT in summer, with ET as the year-round label. Writing "EST" for a July meeting is a common and confusing error, since the clock is on EDT then. Use the converter and name the place if you want no ambiguity at all.