Choosing correct interval for validation examples for time series: Rossman Competition

Hello,

I’m working through the Rossman Competition notebook named lesson3-rossman.ipynb in the dl1/ folder in the fastai library.

Why was the date interval 2014/8/1 - 2014/9/17 in year 2014 chosen for the validation period when the training data goes to 2015-07-31? Why not use the same number of period as the test set but at the end of the training data?

By choosing this time interval for validation and training on the remaining data (which includes dates after the validation period), are we not training on partly on future data?

Thank you