Complicated Excel Exercise. Calculating sales values for each day of the year.

I have 1115 stores for which I need to calculate sales values. The sales values are needed for everyday for the months of January-July for every single store. It is very confusing and almost impossible for me to figure out, which excel function would do such calculation.

Open the following link:

https://www.kaggle.com/c/rossmann-store-sales/data

and download train.csv and test.csv

I only want to use 2015 data to calculate sales values. I need these values for forecasting.
As you will see that there is an unique identifier column named "Id" in the test.csv. That "Id" is
what makes each day unique with its sales, date, day of week, and store.

It is really confusing for me on how to do this. I need to forecast data for August and September 2015.
If anyone knows how to do that as well, please help me on that too.

Thanks!!