[ios] How do you create a Swift Date object?

How do you create a date object from a date in swift xcode.

eg in javascript you would do: var day = new Date('2014-05-20');

This is best done using an extension to the existing NSDate class.

The following extension adds a new initializer which will create a date in the current locale using the date string in the format you specified.

extension NSDate
{
    convenience
      init(dateString:String) {
      let dateStringFormatter = NSDateFormatter()
      dateStringFormatter.dateFormat = "yyyy-MM-dd"
      dateStringFormatter.locale = NSLocale(localeIdentifier: "en_US_POSIX")
      let d = dateStringFormatter.dateFromString(dateString)!
      self.init(timeInterval:0, sinceDate:d)
    }
 }

Now you can create an NSDate from Swift just by doing:

NSDate(dateString:"2014-06-06")

Please note that this implementation does not cache the NSDateFormatter, which you might want to do for performance reasons if you expect to be creating many NSDates in this way.

Please also note that this implementation will simply crash if you try to initialize an NSDate by passing in a string that cannot be parsed correctly. This is because of the forced unwrap of the optional value returned by dateFromString. If you wanted to return a nil on bad parses, you would ideally use a failible initializer; but you cannot do that now (June 2015), because of a limitation in Swift 1.2, so then you're next best choice is to use a class factory method.

A more elaborate example, which addresses both issues, is here: https://gist.github.com/algal/09b08515460b7bd229fa .

Update for Swift 5

extension Date {
    init(_ dateString:String) {
        let dateStringFormatter = DateFormatter()
        dateStringFormatter.dateFormat = "yyyy-MM-dd"
        dateStringFormatter.locale = NSLocale(localeIdentifier: "en_US_POSIX") as Locale
        let date = dateStringFormatter.date(from: dateString)!
        self.init(timeInterval:0, since:date)
    }
}

According to Apple documentation

Example :

var myObject = NSDate()
let futureDate = myObject.dateByAddingTimeInterval(10)
let timeSinceNow = myObject.timeIntervalSinceNow

According to Apple's Data Formatting Guide

Creating a date formatter is not a cheap operation. If you are likely to use a formatter frequently, it is typically more efficient to cache a single instance than to create and dispose of multiple instances. One approach is to use a static variable

And while I agree with @Leon that this should be failable initializer, when you enter seed data, we could have one that isn't failable (just like there is UIImage(imageLiteralResourceName:)).

So here's my approach:

extension DateFormatter {
  static let yyyyMMdd: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateFormat = "yyyy-MM-dd"
    formatter.calendar = Calendar(identifier: .iso8601)
    formatter.timeZone = TimeZone(secondsFromGMT: 0)
    formatter.locale = Locale(identifier: "en_US_POSIX")
    return formatter
  }()
}

extension Date {
    init?(yyyyMMdd: String) {
        guard let date = DateFormatter.yyyyMMdd.date(from: yyyyMMdd) else { return nil }
        self.init(timeInterval: 0, since: date)
    }

    init(dateLiteralString yyyyMMdd: String) {
        let date = DateFormatter.yyyyMMdd.date(from: yyyyMMdd)!
        self.init(timeInterval: 0, since: date)
    }
}

And now enjoy simply calling:

// For seed data
Date(dateLiteralString: "2020-03-30")

// When parsing data
guard let date = Date(yyyyMMdd: "2020-03-30") else { return nil }

In, Swift 3.0 you have set date object for this way.

extension Date
{
    init(dateString:String) {
        let dateStringFormatter = DateFormatter()
        dateStringFormatter.dateFormat = "yyyy-MM-dd"
        dateStringFormatter.locale = Locale(identifier: "en_US_POSIX")
        let d = dateStringFormatter.date(from: dateString)!
        self(timeInterval:0, since:d)
    }
}

Here's how I did it in Swift 4.2:

extension Date {

    /// Create a date from specified parameters
    ///
    /// - Parameters:
    ///   - year: The desired year
    ///   - month: The desired month
    ///   - day: The desired day
    /// - Returns: A `Date` object
    static func from(year: Int, month: Int, day: Int) -> Date? {
        let calendar = Calendar(identifier: .gregorian)
        var dateComponents = DateComponents()
        dateComponents.year = year
        dateComponents.month = month
        dateComponents.day = day
        return calendar.date(from: dateComponents) ?? nil
    }
}

Usage:

let marsOpportunityLaunchDate = Date.from(year: 2003, month: 07, day: 07)

According to @mythz answer, I decide to post updated version of his extension using swift3 syntax.

extension Date {
    static func from(_ year: Int, _ month: Int, _ day: Int) -> Date?
    {
        let gregorianCalendar = Calendar(identifier: .gregorian)
        let dateComponents = DateComponents(calendar: gregorianCalendar, year: year, month: month, day: day)
        return gregorianCalendar.date(from: dateComponents)
    }
}

I don't use parse method, but if someone needs, I will update this post.

Personally I think it should be a failable initialiser:

extension Date {

    init?(dateString: String) {
        let dateStringFormatter = DateFormatter()
        dateStringFormatter.dateFormat = "yyyy-MM-dd"
        if let d = dateStringFormatter.date(from: dateString) {
            self.init(timeInterval: 0, since: d)
        } else {
            return nil
        }
    }
}

Otherwise a string with an invalid format will raise an exception.

I often have a need to combine date values from one place with time values for another. I wrote a helper function to accomplish this.

let startDateTimeComponents = NSDateComponents()
startDateTimeComponents.year = NSCalendar.currentCalendar().components(NSCalendarUnit.Year, fromDate: date).year
startDateTimeComponents.month = NSCalendar.currentCalendar().components(NSCalendarUnit.Month, fromDate: date).month
startDateTimeComponents.day = NSCalendar.currentCalendar().components(NSCalendarUnit.Day, fromDate: date).day
startDateTimeComponents.hour = NSCalendar.currentCalendar().components(NSCalendarUnit.Hour, fromDate: time).hour
startDateTimeComponents.minute = NSCalendar.currentCalendar().components(NSCalendarUnit.Minute, fromDate: time).minute
let startDateCalendar = NSCalendar(identifier: NSCalendarIdentifierGregorian)
combinedDateTime = startDateCalendar!.dateFromComponents(startDateTimeComponents)!

Swift doesn't have its own Date type, but you to use the existing Cocoa NSDate type, e.g:

class Date {

    class func from(year: Int, month: Int, day: Int) -> Date {
        let gregorianCalendar = NSCalendar(calendarIdentifier: .gregorian)!

        var dateComponents = DateComponents()
        dateComponents.year = year
        dateComponents.month = month
        dateComponents.day = day

        let date = gregorianCalendar.date(from: dateComponents)!
        return date
    }

    class func parse(_ string: String, format: String = "yyyy-MM-dd") -> Date {
        let dateFormatter = DateFormatter()
        dateFormatter.timeZone = NSTimeZone.default
        dateFormatter.dateFormat = format

        let date = dateFormatter.date(from: string)!
        return date
    }
}

Which you can use like:

var date = Date.parse("2014-05-20")
var date = Date.from(year: 2014, month: 05, day: 20)