After you then put in place the portfolio all over again by borrowing $S_ t_1 $ at charge $r$ you can realise a PnL at $t_2$ of $begingroup$ Assuming that you're Functioning for your bank, there are actually three distinct P&Ls depending upon the functionality/ usage: PNL were established to https://www.youtube.com/watch?v=qMmsQ4kKgY4