Kolkata and Mumbai are in the same time zone. Both cities use Indian Standard Time (IST, UTC+5:30). When it's noon in Kolkata, it's also noon in Mumbai.
India uses a single time zone across the entire country.
Any time works! Since both cities are in the same time zone, scheduling is never an issue.
Kolkata and Mumbai are India's two largest port cities on opposite coasts. Flights connect them in about 2.5 hours.