So I've noticed over and over again on futures options that the theta value on the day before expiration "stalls", dropping to almost zero the day before the last trading day. Is this a miscalculation on part of IB or is this theta value correct? Yes, I am saying that the theta loss on the 2nd to last trading day is less than the 3rd to last day's theta loss, and the 3rd to last day's is less than the last day's. All the charts I've seen that show theta as a function of time til expiration show a graph that contradicts this behavior. Also, I am aware that implied volatilities tend to drop as expiration approaches, but this is a totally different phenomenon. So is IB miscalculating this?
I don't know what IB does on their TWS platform. I can tell you that some use days to expiration as whole days and some factor in the fraction of a day. Eg, some might always show 2 days left and others might show 1.5 days. Using the fraction of the day is more accurate for your greeks.