你如何创建一个 Swift Date 对象？

Question

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');

Answer 1

Swift has its own Date type. No need to use NSDate .

Creating a Date and Time in Swift

In Swift, dates and times are stored in a 64-bit floating point number measuring the number of seconds since the reference date of January 1, 2001 at 00:00:00 UTC . This is expressed in the Date structure . The following would give you the current date and time:

let currentDateTime = Date()

For creating other date-times, you can use one of the following methods.

Method 1

If you know the number of seconds before or after the 2001 reference date, you can use that.

let someDateTime = Date(timeIntervalSinceReferenceDate: -123456789.0) // Feb 2, 1997, 10:26 AM

Method 2

Of course, it would be easier to use things like years, months, days and hours (rather than relative seconds) to make a Date . For this you can use DateComponents to specify the components and then Calendar to create the date. The Calendar gives the Date context. Otherwise, how would it know what time zone or calendar to express it in?

// Specify date components
var dateComponents = DateComponents()
dateComponents.year = 1980
dateComponents.month = 7
dateComponents.day = 11
dateComponents.timeZone = TimeZone(abbreviation: "JST") // Japan Standard Time
dateComponents.hour = 8
dateComponents.minute = 34

// Create date from components
let userCalendar = Calendar(identifier: .gregorian) // since the components above (like year 1980) are for Gregorian
let someDateTime = userCalendar.date(from: dateComponents)

Other time zone abbreviations can be found here . If you leave that blank, then the default is to use the user's time zone.

Method 3

The most succinct way (but not necessarily the best) could be to use DateFormatter .

let formatter = DateFormatter()
formatter.dateFormat = "yyyy/MM/dd HH:mm"
let someDateTime = formatter.date(from: "2016/10/08 22:31")

The Unicode technical standards show other formats that DateFormatter supports.

Notes

See my full answer for how to display the date and time in a readable format. Also read these excellent articles:

• How to work with dates and times in Swift 3, part 1: Dates, Calendars, and DateComponents
• How to work with dates and times in Swift 3, part 2: DateFormatter
• How to work with dates and times in Swift 3, part 3: Date arithmetic

Answer 2

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 NSDate s 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)
    }
}

Answer 3

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

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)

Answer 4

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)

Answer 5

According to Apple documentation

Example :

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

Answer 6

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)
    }
}

Answer 7

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.

Answer 8

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.

Answer 9

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)!

Answer 10

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 preferably be a failable initializer, it doesn't always have to be (just like there is UIImage(imageLiteralResourceName:) that will crash if the resource doesn't exist).

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 cases where the date exists for sure (eg. seed data)
Date(dateLiteralString: "2020-03-30")

// The rest of the time (eg. parsing unknown data)
guard let date = Date(yyyyMMdd: "2020-03-30") else { return nil }