Test for goodness of fit of a time series

Hello, I've been recently working on a fractal regression model that fits to the sp500 index. So, this index is a time series which I'm not very familiar with, and I wanted to test the goodness of fit of my function over this data. I've been suggested to try the 2-sample Kolmogorov Smirnov test, is it the ideal test? Also I'm working on Excel, not Rstudio, so if you have any tutorials or papers about any tests I should run, …

data datascience fractal function index regression series test time series