After you then create the portfolio all over again by borrowing $S_ t_1 $ at charge $r$ you are able to realise a PnL at $t_2$ of $begingroup$ For an option with selling price $C$, the P$&$L, with regard to improvements with the underlying asset price $S$ and volatility $sigma$, https://www.youtube.com/watch?v=qMmsQ4kKgY4