The Area/Location identifiers — Europe/London, Asia/Kolkata — that carry a zone's whole rule history.
The IANA Time Zone Database gives every zone an identifier of the form Area/Location: Europe/London, America/New_York, Asia/Kolkata. Nearly every operating system and programming language uses it, which makes it the closest thing to a universal vocabulary for time.
The identifier is not just a label for a current offset. It carries the zone's whole history of rules, so a date in 1968 converts using the rules of 1968. That is why storing an identifier rather than an offset is the difference between a system that keeps working and one that quietly stops.
Identifiers are chosen to be stable rather than politically current, and the database keeps old names working as aliases. Asia/Calcutta still resolves; the canonical name is Asia/Kolkata.
IANA identifiers are unique and stable. Abbreviations like IST are neither, and mean at least three different things.
An identifier resolves to an offset at a given instant. Storing the offset instead throws away everything needed to do it again later.
See also time zone · utc offset · time zone abbreviation · daylight saving time
We hold this as the timezone field is an IANA identifier for every jurisdiction. See what we hold.