CST to IST

How far apart Chicago and Kolkata really are

The difference between these two zones is not a single number. Across 2026 it takes 2 different values — 10 hours 30 minutes, 11 hours 30 minutes — because Chicago observes daylight saving and Kolkata does not.

Working-hours overlap

There is no overlap between a 09:00–17:00 day in Chicago and the same window in Kolkata — the zones are 10 hours 30 minutes apart. A recurring call has to sit in one side's early morning or late evening, so the fair arrangement is usually to alternate which side absorbs it.

Why you should store the zone, not the offset

Because the gap between Chicago and Kolkata changes during the year, an offset recorded as a number is only correct until the next transition. Store America/Chicago and Asia/Kolkata — the IANA identifiers — alongside any time a user entered locally. The identifier carries the zone's entire history of rule changes, so a date from 1995 converts with the rules that were in force in 1995.

Internally, keep the instant as a Unix timestamp. The epoch value is identical in Chicago, Kolkata and everywhere else; the zone is applied only at the edges, when a human needs to read it. The current Unix timestamp is at the top of this page, and the zones versus offsets guide explains why the distinction bites.