During World War II, all over pre-independence India, i.e. present day India, Pakistan and Bangladesh, clocks were moved forward by one hour to make Indian time 6 hours and 30 minutes ahead of GMT. The exact dates for this change are from September 1, 1942 to October 14, 1945. 1
For any charts prepared for births during this period, simply move the clock back for an hour -OR- fill 6 30 E in the timezone column. E.g. for a chart prepared for Jan 01, 1944, Bombay at 9:00 AM, both of the following are valid inputs for making a birth chart:
Both these inputs in the birth chart generation application essentially give us the same Vedic Astrology birth chart –
1As suggested by Basab, I did some research on astro.com website. If I take a date of 15th August, 1947 and cast a chart for 12:00 AM at New Delhi, the software on the site automatically suggests IST, i.e. GMT+5:30. If the city is changed to Calcutta, the software suggests IDT, which is GMT+6:30. This is indeed confusing. IMHO, in case of a confusion, two horoscopes with GMT+6:30 and GMT+5:30 should be cast and basic chart rectification techniques should be used.