Creating a date with DateComponents

Question

I want to get the first day and the last day of the week. But my results do not match the documentation from apple: https://developer.apple.com/documentation/foundation/nsdatecomponents/1410442-weekday

This is my function:

func startAndEndDateOfWeek(weekOfYearWithYear: (week: Int,year: Int)) -> (start: Date, end: Date) {
    var output = (start: Date.init(), end: Date.init())

    let calendar = Calendar(identifier: .gregorian)
    var firstDayComponents = DateComponents()
    firstDayComponents.weekOfYear = weekOfYearWithYear.week
    firstDayComponents.yearForWeekOfYear = weekOfYearWithYear.year
    firstDayComponents.weekday = 1

    let firstDay = calendar.date(from: firstDayComponents)

    var lastDayComponents = DateComponents()
    lastDayComponents.weekOfYear = weekOfYearWithYear.week
    lastDayComponents.yearForWeekOfYear = weekOfYearWithYear.year
    lastDayComponents.weekday = 2

    let lastDay = calendar.date(from: lastDayComponents)
    output = (start: firstDay!, end: lastDay!)
    return output
}

.weekday = 2 -> leads to the sunday and not 0.

I also want to have the entire day and not 16:00.

A `Date` is always printed in UTC – see https://stackoverflow.com/questions/39937019/nsdate-or-date-shows-the-wrong-time. You are probably in the GMT+8 timezone. — Martin R, Mar 01 '19 at 05:56
okay makes sense, but do you have any idea why creating the date with the dayComponents does not really work? — PaFi, Mar 01 '19 at 06:08

Rob · Accepted Answer · 2019-03-01T16:31:45.803

A couple of observations:

In the Gregorian calendar, weekday = 1 means Sunday; weekday = 2 means Monday; etc. You can look at calendar.maximumRange(of: .weekday) to get the range of valid values, and you can look at calendar.weekdaySymbols to see what these weekDay values mean (e.g. “Sun”, “Mon”, “Tue”, “Wed”, “Thu”, “Fri”, and “Sat”).

You said:

I also want to have the entire day and not 16:00.

A Date object references a moment in time. So it can’t represent an “entire day”. But it can represent midnight (and midnight in your time zone is likely 4pm in GMT/UTC/Zulu).

You can, alternatively, return a DateInterval, which does represent a range of time.

func interval(ofWeek week: Int, in year: Int) -> DateInterval {
    let calendar = Calendar.current

    let date = DateComponents(calendar: calendar, weekOfYear: week, yearForWeekOfYear: year).date!
    return calendar.dateInterval(of: .weekOfYear, for: date)!
}

And then

let formatter = DateIntervalFormatter()
formatter.dateStyle = .short
formatter.timeStyle = .short

let year = Calendar.current.component(.year, from: Date())
let dateInterval = interval(ofWeek: 2, in: year)
print(formatter.string(from: dateInterval))

In a US locale, the interval starts on January 6th:

1/6/19, 12:00 AM – 1/13/19, 12:00 AM

Whereas in a German locale, the interval starts on the 7th:

07.01.19, 00:00 – 14.01.19, 00:00

If you want the start of the first day of the week and the last day of the week, you can do:

func startAndEndDate(ofWeek week: Int, in year: Int) -> (Date, Date) {
    let date = DateComponents(calendar: calendar, weekOfYear: week, yearForWeekOfYear: year).date!
    let lastDate = calendar.date(byAdding: .day, value: 6, to: date)!

    return (date, lastDate)
}

And then

let formatter = DateFormatter()
formatter.dateStyle = .short

let year = Calendar.current.component(.year, from: Date())
let (start, end) = startAndEndDate(ofWeek: 2, in: year)
print(formatter.string(from: start), "-", formatter.string(from: end))

Setting `weekday = weekDayRange?.first (== 1)` does not take into account that a week starts on a Monday in some regions (like Germany). It might be better *not* to set that component for the first date, and compute the second date by adding one week minus one day. — Martin R, Mar 01 '19 at 06:15
if I use the users current Calendar like Calendar.current, it does not handle the users region right? so basically I have to handle all the various calendar the user might use? Or what would be a more general solution? — PaFi, Mar 01 '19 at 06:23
@MartinR - You’re right. I’ve updated my answer accordingly — Rob, Mar 01 '19 at 07:19
@PaFi - No, I’d definitely suggest always using `Calendar.current`. Someone using a non-gregorian calendar will see very unusual results, if you don’t. Definitely test your code using non-gregorian calendars and different locales. — Rob, Mar 01 '19 at 07:22
A German user will see "07.01.19, 00:00 – 14.01.19, 00:00". `Calendar` and `DateIntervalFormatter` have a `locale` property, the default value (probably) is the current locale which can be modified in the system prefs. — Willeke, Mar 01 '19 at 09:40
@Willeke - Of course it will. I was focusing on Martin’s point, how the definition of of the week changes from locale to locale. But obviously, the formatting of the formatter’s resulting string also varies. — Rob, Mar 01 '19 at 16:33

Creating a date with DateComponents

1 Answers1