India’s stock markets tried something new on August 3. It did not go smoothly.
The National Stock Exchange and Bombay Stock Exchange launched their Closing Auction Session, a 20-minute auction window designed to replace the old method of calculating end-of-day prices. The result was immediate confusion among traders and an unexpected spike in the Nifty 50 Index as participants scrambled to understand the new price discovery mechanics.
What actually changed
For years, India’s exchanges calculated official closing prices using a volume-weighted average price (VWAP) method across a 30-minute window.
The new system flips that approach entirely. Instead of averaging, the CAS creates a dedicated auction period running from 3:15 PM to 3:35 PM where buyers and sellers submit orders, and a single equilibrium price emerges.
