Yuyu,
That's correct. Day Traders have "T" or "T+1" for "Contra-Settlement".
Closing out on T+2 or T+3 means that funds are required to complete.
I suspect that this is the real reason that so many shares trade hands in the last 30 minute of play.
Day traders must cop the loss or take the profit no matter what.....unless they have ample funds.