以下是我在Swift 4.2中是如何做到的:

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
    }
}

用法:

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

Swift没有自己的Date类型，但是你可以使用现有的Cocoa NSDate类型，例如:

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
    }
}

你可以这样用:

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

在Swift 3.0中，您已经为此设置了日期对象。

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)
    }
}

根据苹果的数据格式指南

创建日期格式化程序并不是一项简单的操作。如果你有可能 要经常使用格式化程序，缓存通常更有效 单个实例比创建和处理多个实例更容易。 一种方法是使用静态变量

虽然我同意@Leon，这应该是一个可失败的初始化器，它并不总是必须的(就像有UIImage(imageLiteralResourceName:)，将崩溃，如果资源不存在)。

这是我的方法:

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)
    }
}

现在享受简单的呼叫:

// 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 }

我经常需要将一个地方的日期值与另一个地方的时间值结合起来。我写了一个辅助函数来完成这个任务。

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)!

我个人认为它应该是一个失败的初始化器:

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
        }
    }
}

否则，格式无效的字符串将引发异常。