call_put_flag: 'c' for a call and 'p' for a put
spot: spot price
strike: strike price
time: time to maturity in years
rate: risk-free interest rate to the exercise date in percent
volatility: annualized volatility of the asset in percent for the period through to the exercise date
cost_of_carry: net cost of holding the underlying asset (for common stocks, the risk free rate less the dividend yield), defaults to 0
OPT_BS_THETA uses the Black-Scholes model to calculate the 'theta' of a European option struck at strike on an asset with spot price spot. The theta of an option is the rate of change of its price with respect to time to expiry.
Theta is expressed as the negative of the rate of change of the option value, per 365.25 days.