Hi Philippe – The format for the date range you looks correct. Here is a simple example where Fit X range is set to 2000:1:1,2000:2:1. As a result, the linear fit only uses the three highlighted points in that range.
So maybe something else is not right. Is your data set to the correct date format? Another possibility is if you are using a LOESS fit you may have to increase the smoothing window in order for the fit to work.