Swift - Problem converting year + weekOfYear components to Date [duplicate] - swift

Question

When I run this code:
let calendar = Calendar.current
var dateComponents = DateComponents()
dateComponents.weekday = calendar.firstWeekday
dateComponents.weekOfYear = 2
dateComponents.year = 2017
let startOfWeek = calendar.date(from: dateComponents)
let endOfWeek = calendar.date(byAdding: .day, value: 6, to: startOfWeek!)
let formatter = DateFormatter()
formatter.dateStyle = .short
print(formatter.string(from: startOfWeek!))
print(formatter.string(from: endOfWeek!))
It prints this:
1/8/17
1/14/17
When I change the code to this:
dateComponents.weekOfYear = 1
dateComponents.year = 2017
It prints this:
12/31/17
1/6/18
Why is it 12/31/17?

Answer 1

When I use .full style to print the dates, I get Sunday, December 31, 2017 for the first date, but it's obviously wrong because December 31 is a Thursday.
If you want to get the correct date, use yearForWeekOfYear instead of year. Docs:
You can use the yearForWeekOfYear property with the weekOfYear and weekday properties to get the date corresponding to a particular weekday of a given week of a year. For example, the 6th day of the 53rd week of the year 2005 (ISO 2005-W53-6) corresponds to Sat 1 January 2005 on the Gregorian calendar.
Alternative, you can be a little naughty and not listen to the docs and use weekOfYear = 54:
let calendar = Calendar.current
var dateComponents = DateComponents()
dateComponents.weekday = calendar.firstWeekday
dateComponents.weekOfYear = 54
dateComponents.year = 2017
let startOfWeek = calendar.date(from: dateComponents)
let endOfWeek = calendar.date(byAdding: .day, value: 6, to: startOfWeek!)
let formatter = DateFormatter()
formatter.dateStyle = .short
print(formatter.string(from: startOfWeek!))
print(formatter.string(from: endOfWeek!))
This prints:
1/1/17
1/7/17
which is coincidentally, the correct dates.