Model fit behaves very differently between `datetime64[ns]` and `datetime64[ms]`.

I was unknowingly passing a dtype of `datetime64[ms]` to prophet and getting a _very_ poor fit. Converted back to `datetime64[ns]` and model fit behaved as expected. It might be worth adding a check to warn other users.

My approach to convert was: `df['ds'] = pd.to_datetime(df['ds'].dt.strftime('%Y-%m-%d'))`

NB: A possible cause of getting the `[ms]` type is converting from a polars frame.