I created a Default Time Series model with a Derivation Window = -35.
But now when I look at the Prediction it says the Historical Rows = -42.
Cannot figure what is going on here. Should they not be the same?
It sounds like you have a weekly seasonality in your data. The number of historical rows is calculated by adding the longest periodicity in your dataset to the feature derivation window. From the docs:
"In this example, the prediction dataset needs at least 42 days of historical data and can predict (return) up to 7 rows. That is because although the model was configured for 35 days before the forecast point, seven days are added to the required history because the model uses seven-day differencing. Generally, Historical rows = FDW size + seasonality
, where seasonality is the longest periodicity detected."