I am trying to obtain the Theta from Closed Formula by using Finite Difference methods and I observe some discrepancies. For instance, here with the following parameters:
Spot:50, Strike:50, Rate: 0.12 Time To maturity: 0.25, Volatility: 0.3
BS Closed Form: -8.528247797676
BS Forward FD: -5.8503941392455516
I applied a change of -1/365 in T to compute the BS Forward FD.
Please note that I am perfectly able to match Delta, Gamma and Vega. I don’t know what is wrong with Theta. Any idea?