You don't assign the output of the first line to anything, so it's a wasted lines. The second line just call dir again but since you don't specify a directory, does it in the current directory (which, it looks like, has 17 files/folders).
Avoid using the command form of functions in scripts. Here is a clean way of doing it:
path = 'C:\User\Jason\MATLAB\Days';
dircontent = dir(path);
numfolders = sum([dircontent.isdir]) - 2; %-2 to account for the stupid '.' and '..' returned by dir
