From August 3rd, exchanges in India are changing how closing prices are determined for stocks in the futures and options (F&O) segment. Previously, a stock's closing price was based on the average price of trades conducted in the last 30 minutes, between 3 PM and 3:30 PM. The new system introduces a "Closing Auction Session" (CAS) for F&O stocks, which will decide their official closing price. This move by the Securities and Exchange Board of India (SEBI) and the National Stock Exchange (NSE) is intended to rein in manipulation and improve the efficiency of large order execution.
Under the new rules, continuous trading for F&O stocks will end at 3:15 PM. Following this, a 20-minute closing auction will commence. During this period, buy and sell orders are collected but not immediately executed. Instead, the exchanges will calculate a single equilibrium price at which the maximum number of buy and sell orders can be matched. This price will then become the stock's official closing price. This mechanism is designed to make the closing price more reliable and reduce the impact of sudden, large orders placed near the end of the trading day that could influence the final price.
The rationale behind this change is to address concerns about large orders being placed at the close of trading to artificially influence the final price, which can impact closing levels, mutual fund portfolios, and derivatives contracts. The auction will also close at a random time between 3:28 PM and 3:30 PM, making it harder for traders to time last-second orders. While F&O stocks undergo this auction process, other stocks not part of the F&O segment will continue to use the existing 30-minute volume-weighted average price (VWAP) method until 3:30 PM.
For active traders, proprietary desks, and institutions trading near market close, the impact will be significant. Brokers may advance intraday square-off timings, necessitating traders to check revised cut-off times. Investors in non-F&O stocks, however, are unlikely to notice much difference. The reference price for the auction is determined by the VWAP of trades between 3:00 PM and 3:15 PM, and a price band of +/- 3% from this reference price is applicable during the CAS to curb excessive price movements.
