Kolkata, Mumbai had own time zones even after IST
Kolkata and Mumbai continued to follow their own time zone even after the Indian Standard Time (IST) was declared as the country's official time. Calcutta Time (UTC +5:54) and Bombay Time (UTC +4:51) lasted till 1948 and 1955 respectively. The Indian government declared IST (UTC +5:30) as the official time 69 years ago on September 1, 1947.