Creating function in R that processes rasters in Esri grid format?

I have loads of data (6500 files that I need to process and find yearly winter sums/averages/variance for. the format for my naming system is Syyyymmdd_rp. Each file is in the same file folder. I wanted to create a function that i could put a date and the statistical methods for calculations and I am having trouble starting. Keep in mind that the winter of 2014 spans back to September 2013. So essentially I want a function that I can put a start and end date into, as well as what method to calculate to rasters with (mean, sum, variance).