Getting unexpected date from day formatter using weekOfYear-CodePudding

I'm trying to get the start of the week using the current week and my return value is a week off. For example, if I input weekNumber 17 I would expected April 25th to be returned based off this, but I'm getting back April 17th.

I did check out the following post and that is where I got the function below. get first day of week using Weeknumber in Swift

func getFirstDay(WeekNumber weekNumber:Int, CurrentYear currentYear: Int)->String?{
    let Calendar = NSCalendar(calendarIdentifier: .gregorian)!
    var dayComponent = DateComponents()
    dayComponent.weekOfYear = weekNumber
    dayComponent.weekday = 1
    dayComponent.year = currentYear
    var date = Calendar.date(from: dayComponent)

    if(weekNumber == 1 && Calendar.components(.month, from: date!).month != 1){
        dayComponent.year = currentYear - 1
        date = Calendar.date(from: dayComponent)
    }
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "yyyy/MM/dd"
    return String(dateFormatter.string(from: date!))
}

CodePudding user response：

There are a few issues

For week numbers use always the ISO8601 calendar.
For week numbers the correct year property is yearForWeekOfYear.
In your code the week starts on Sunday (1), the linked site refers to Monday.
In Swift use native Calendar.
In the last line the String initializer is redundant.
Don't force unwrap dates.

func getFirstDay(weekNumber:Int, currentYear: Int) -> String? {
    let calendar = Calendar(identifier: .iso8601)
    var dayComponent = DateComponents()
    dayComponent.weekOfYear = weekNumber
    dayComponent.yearForWeekOfYear = currentYear
    guard let date = calendar.date(from: dayComponent) else { return nil }
    let dateFormatter = DateFormatter()
    dateFormatter.locale = Locale(identifier: "en_US_POSIX")
    dateFormatter.dateFormat = "yyyy/MM/dd"
    return dateFormatter.string(from: date)
}