Nine and a half hours, or ten and a half
India Standard Time sits at UTC+5:30 and never changes: no daylight saving, one zone for the whole country. Eastern does change. That means the gap between New York and Delhi is 9 hours 30 minutes during American daylight saving, and 10 hours 30 minutes for the winter months. The difference moves twice a year even though nothing changes in India.
The half hour that breaks mental arithmetic
India is one of a handful of places on a thirty-minute offset, so answers land on the half hour: 9:00 AM in New York is 6:30 PM in Mumbai. People used to whole-hour conversions routinely drop the thirty minutes and book a call that misses by half an hour. Nepal is worse still, at UTC+5:45.
Finding a workable slot
The overlap is narrow. Morning in New York is evening in India: an 8:00 AM Eastern call is 5:30 PM or 6:30 PM in Bengaluru, which is the end of a working day but survivable. Anything after about 10:00 AM Eastern pushes India past 8:00 PM. In the other direction, the Indian morning is the American middle of the night. Most teams settle on the 7:30 to 9:30 AM Eastern band and accept that one side is finishing late.
IST is an overloaded abbreviation
IST means India Standard Time, but also Irish Standard Time at UTC+1 and Israel Standard Time at UTC+2. In an email thread spanning several countries, write the city. The three letters are not enough.