Your code is working perfectly fine. If you would run your code in a Playground, you could see the actual Date objects rather than there String representations. Even if you don't specify a DateFormatter, under the hood, when doing print(Date()), Swift uses a DateFormatter to print Date objects and hence that value will be a relative time instead of the absolute point in time provided by the Date object.
In a Playground you can see that the actual startOfWeek is "Sep 25, 2017 at 12:00 AM", while the endOfWeek is "Oct 1, 2017 at 12:00 AM", which are the correct values for the current week.
Don't use print(Date()) for checking the correctness of Date objects, since as you can see, it will yield unexpected results due to the underlying DateFormatters used.
