Get the First Day of Week Without Weekcalendarunit

Get the first day of week without WeekCalendarUnit

You can use .CalendarUnitWeekOfYear in combination with
.CalendarUnitYearForWeekOfYear, this should give the intended result:

let currentDateComponents = calendar.components(.CalendarUnitYearForWeekOfYear | .CalendarUnitWeekOfYear , fromDate: date)
let startOfWeek = calendar.dateFromComponents(currentDateComponents)

Alternatively, use rangeOfUnit() (which I personally find easier to
use):

var startOfWeek : NSDate?
calendar.rangeOfUnit(.CalendarUnitWeekOfYear, startDate: &startOfWeek, interval: nil, forDate: date)

In Swift 2 this has to be changed to

let currentDateComponents = calendar.components([.YearForWeekOfYear, .WeekOfYear ], fromDate: date)
let startOfWeek = calendar.dateFromComponents(currentDateComponents)

and

var startOfWeek : NSDate?
calendar.rangeOfUnit(.WeekOfYear, startDate: &startOfWeek, interval: nil, forDate: date)

get first day of week using Weeknumber in Swift

    func getFirstDay(weekNumber:Int)->String?{
        let Calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
        let dayComponent = NSDateComponents()
        dayComponent.weekOfYear = weekNumber
        dayComponent.weekday = 1
        dayComponent.year = 2015
        var date = Calendar.dateFromComponents(dayComponent)

        if(weekNumber == 1 && Calendar.components(.Month, fromDate: date!).month != 1){
            print(Calendar.components(.Month, fromDate: date!).month)
            dayComponent.year = 2014
            date = Calendar.dateFromComponents(dayComponent)
        }
        let dateFormatter = NSDateFormatter()
        dateFormatter.dateFormat = "dd-MM-yyyy"
        return String(dateFormatter.stringFromDate(date!))
    }

Very rough code what i did so far.

It works for finding the first day of the week ;)!

Date from week of year returning date not in week

Changing the firstWeekday from 1 to 2 won't change the date, it will change just the First weekday from Sunday to Monday.

You can do it like this:

func dateFromWeekOfYear(year:Int, weekOfYear:Int, weekday:Int) -> NSDate {
    return NSCalendar.currentCalendar().dateWithEra(1, yearForWeekOfYear: year, weekOfYear: weekOfYear, weekday: weekday, hour: 0, minute: 0, second: 0, nanosecond: 0)!
}

let date1 = dateFromWeekOfYear(2014, 52, 1)   // Dec 21, 2014, 12:00 AM
let date2 = dateFromWeekOfYear(2014, 52, 2)   // Dec 22, 2014, 12:00 AM
let date3 = dateFromWeekOfYear(2014, 52, 3)   // Dec 23, 2014, 12:00 AM

If dealing with a string and you want to set he Stand Alone local day of week you can do it like this:

let myDate = "2 52 2014"
let cal = NSCalendar.currentCalendar()
let formatter = NSDateFormatter()
formatter.dateFormat = "c ww Y"
formatter.calendar = cal
if let date1 = formatter.dateFromString(myDate) {
    date1  // "Dec 22, 2014, 12:00 AM"
}

If you need further reference you can use this:

List of Days of this week

Edit: missed that you want a week to always start on a Monday

let gregorian = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
let today = gregorian.startOfDayForDate(NSDate())

let startOfWeek: NSDate    
if gregorian.component(.Weekday, fromDate: today) == 2 {
    // Today is Monday
    startOfWeek = today
} else {
    // Find the last Monday prior to today
    startOfWeek = gregorian.nextDateAfterDate(today, matchingUnit: .Weekday, value: 2, options: [.SearchBackwards, .MatchPreviousTimePreservingSmallerUnits])!
}

Get a Week From a Different Starting Day

You can set the firstWeekday property of the current calendar to determine
the first day of the week for all calendrical calculations:

let cal = NSCalendar.currentCalendar()
cal.firstWeekday = 5 // 1 == Sunday, 5 == Thursday

Use rangeOfUnit to determine the start and of the week containing the chosen date:

let chosenDate = ...
var startOfWeek : NSDate? = nil
var lengthOfWeek : NSTimeInterval = 0
cal.rangeOfUnit(.WeekCalendarUnit, startDate: &startOfWeek, interval:&lengthOfWeek, forDate: chosenDate)
var endOfWeek = startOfWeek!.dateByAddingTimeInterval(lengthOfWeek)

Iterate over the dates:

var date = startOfWeek!
let formatter = NSDateFormatter()
formatter.dateFormat = "EEEE, dd.MM.yyyy"

while date.compare(endOfWeek) == NSComparisonResult.OrderedAscending  {
    println(formatter.stringFromDate(date))
    date = cal.dateByAddingUnit(.DayCalendarUnit, value: 1, toDate: date, options: NSCalendarOptions(0))
}

Example output (for today as the chosen date):

Thursday, 31.07.2014
Friday, 01.08.2014
Saturday, 02.08.2014
Sunday, 03.08.2014
Monday, 04.08.2014
Tuesday, 05.08.2014
Wednesday, 06.08.2014

Get day of week using NSDate

What you are looking for (if I understand the question correctly) is NSCalendarUnit.CalendarUnitWeekday. The corresponding property of NSDateComponents is weekday.

Note also that your date format is wrong (the
full specification can be found here: http://unicode.org/reports/tr35/tr35-6.html).

The function can be simplified slightly, using automatic type inference, also you use variables a lot where constants are sufficient.
In addition, the function should return an optional which is nil
for an invalid input string.

Updated code for Swift 3 and later:

func getDayOfWeek(_ today:String) -> Int? {
    let formatter  = DateFormatter()
    formatter.dateFormat = "yyyy-MM-dd"
    guard let todayDate = formatter.date(from: today) else { return nil }
    let myCalendar = Calendar(identifier: .gregorian)
    let weekDay = myCalendar.component(.weekday, from: todayDate)
    return weekDay
}

Example:

if let weekday = getDayOfWeek("2014-08-27") {
    print(weekday)
} else {
    print("bad input")
}

Original answer for Swift 2:

func getDayOfWeek(today:String)->Int? {

    let formatter  = NSDateFormatter()
    formatter.dateFormat = "yyyy-MM-dd"
    if let todayDate = formatter.dateFromString(today) {
        let myCalendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
        let myComponents = myCalendar.components(.Weekday, fromDate: todayDate)
        let weekDay = myComponents.weekday
        return weekDay
    } else {
        return nil
    }
}

NSDateComponents: difference between weekOfYear and weekOfMonth

The difference between the two show show the week relative to the month or year.

The second week (weekOfMonth) of February would be the seventh week (weekOfYear) of the year, for example, depending on the year.

As to using date components for adding weeks to a date, it doesn't appear to matter which is used but weekOfYear seems to be preferred on the Internet.

I did some testing in a Swift playground and they always calculated the same value when adding weeks to a date.

NSDateComponents returns wrong weekday

It's all correct. If you look at Apple Documentation.
you can see that 1 is Sunday, 2 - is Monday and so forth. So 5 is Thursday as expected.

Error extra argument in call when passing argument to method

Try this:

let now = NSDate()
var startDate: NSDate? = nil
var duration: NSTimeInterval = 0
let cal = NSCalendar.currentCalendar()

cal.rangeOfUnit(NSCalendarUnit.WeekCalendarUnit, startDate: &startDate,
    interval: &duration, forDate: now)

(By default, the first parameter has no external name and thus should not be named.)

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