Trading of QQQQ ends at 4:00PM but the SPDR (SPY) ends at 4:15PM. For MOC orders (market-on-close), different closing time does make difference because price sometimes moves a lot between 4:00 - 4:15. What determines a stock's closing time? QQQQ and SPY are traded on multiple platforms. Which one (NYSE? AMEX? NASDAQ?) makes the decision? And what are the considerations as to whether to close at 4:00 or 4:15? Anybody would like to share some thoughts? Thanks.