What index and web site do you follow to track the S Fund?
TSP.gov states "The S Fund's investment objective is to match the performance of the Dow Jones U.S. Completion Total Stock Market Index, a broad market index made up of stocks of U.S. companies not included in the S&P 500 Index." So that would be DWCPF.
MarketWatch.com shows this index reporting 1pm and ending 8pm Eastern Time. And it does not always match S fund performance, sometimes not even close. Today, it shows DWCPF is down 1% but S fund is up a bit. https://www.marketwatch.com/investing/index/dwcpf?countrycode=xx
Other Web Sites show DWCPF reporting 9:30am-4pm and show returns more close to S fund performance. For example - https://www.google.com/search?tbm=fin&q=dwcpf#scso=_-iX9XueDCsKrytMPxu2dmAE1:0
Thanks in advance if you can clear up this confusion.