Kolkata is the namesake of the IANA zone Asia/Kolkata and uses IST (UTC+05:30).
Detecting your timezone for a personal comparison…
Now
Eastern India’s largest city sees earlier sunrises than Mumbai on the same civil clock. The IANA identifier still uses the older English spelling Calcutta. Kolkata time is identical to every other Indian city page on this site.
Kolkata uses Asia/Kolkata. The current offset is UTC+05:30. Clocks on this page update from that IANA zone rather than from the server’s local time.
Kolkata, India is in India Standard Time (Asia/Kolkata). Kolkata is the namesake of the IANA zone Asia/Kolkata and uses IST (UTC+05:30).
This time zone does not observe daylight saving time India does not observe daylight saving time. IST remains UTC+05:30 every day of the year.
At this moment the offset difference is +00:00 hours relative to UTC labels, meaning Kolkata compared with Mumbai can be converted in the time zone converter using Asia/Kolkata and Asia/Kolkata. Daylight saving on either side is applied automatically.