在Swift 2.2中

    /// Returns the amount of years from another date
func years(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components([.Year], fromDate: fromdate, toDate: NSDate(), options: []).year ?? 0
}
/// Returns the amount of months from another date
func months(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components([.Month], fromDate: fromdate, toDate: NSDate(), options: []).month ?? 0
}
/// Returns the amount of weeks from another date
func weeks(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components([.WeekOfYear], fromDate: fromdate, toDate: NSDate(), options: []).weekOfYear ?? 0
}
/// Returns the amount of days from another date
func days(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components([.Day], fromDate: fromdate, toDate: NSDate(), options: []).day ?? 0
}
/// Returns the amount of hours from another date
func hours(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components([.Hour], fromDate: fromdate, toDate: NSDate(), options: []).hour ?? 0
}
/// Returns the amount of minutes from another date
func minutes(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components([.Minute], fromDate: fromdate, toDate: NSDate(), options: []).minute ?? 0
}
/// Returns the amount of seconds from another date
func seconds(fromdate: NSDate) -> Int {
    return NSCalendar.currentCalendar().components(.Second, fromDate: fromdate, toDate: NSDate(), options: []).second ?? 0
}

使用Swift 3，根据您的需要，您可以选择以下两种方式之一来解决您的问题。

1. 向用户显示两个日期之间的差异

你可以使用DateComponentsFormatter为你的应用程序界面创建字符串。DateComponentsFormatter有一个maximumUnitCount属性，声明如下:

var maximumUnitCount: Int { get set }

使用此属性可限制结果字符串中显示的单位数量。例如，将此属性设置为2，而不是“1h 10m, 30s”，生成的字符串将是“1h 10m”。当空间受限或希望将值舍入到最近的大单位时，请使用此属性。

通过将maximumUnitCount的值设置为1，可以保证仅以一个DateComponentsFormatter的单位(年、月、日、小时或分钟)显示差异。

下面的Playground代码显示了如何显示两个日期之间的差异:

import Foundation

let oldDate = Date(timeIntervalSinceReferenceDate: -16200)
let newDate = Date(timeIntervalSinceReferenceDate: 0)

let dateComponentsFormatter = DateComponentsFormatter()
dateComponentsFormatter.allowedUnits = [NSCalendar.Unit.year, .month, .day, .hour, .minute]
dateComponentsFormatter.maximumUnitCount = 1
dateComponentsFormatter.unitsStyle = DateComponentsFormatter.UnitsStyle.full
let timeDifference = dateComponentsFormatter.string(from: oldDate, to: newDate)

print(String(reflecting: timeDifference)) // prints Optional("5 hours")

注意，DateComponentsFormatter对结果进行四舍五入。因此，4小时和30分钟的差异将显示为5小时。

如果你需要重复这个操作，你可以重构你的代码:

import Foundation

struct Formatters {

    static let dateComponentsFormatter: DateComponentsFormatter = {
        let dateComponentsFormatter = DateComponentsFormatter()
        dateComponentsFormatter.allowedUnits = [NSCalendar.Unit.year, .month, .day, .hour, .minute]
        dateComponentsFormatter.maximumUnitCount = 1
        dateComponentsFormatter.unitsStyle = DateComponentsFormatter.UnitsStyle.full
        return dateComponentsFormatter
    }()

}

extension Date {
    
    func offset(from: Date) -> String? {
        return Formatters.dateComponentsFormatter.string(from: oldDate, to: self)
    }
    
}

let oldDate = Date(timeIntervalSinceReferenceDate: -16200)
let newDate = Date(timeIntervalSinceReferenceDate: 0)

let timeDifference = newDate.offset(from: oldDate)
print(String(reflecting: timeDifference)) // prints Optional("5 hours")

2. 在不设置格式的情况下获取两个日期之间的差异

如果不需要将两个日期之间的差异格式化显示给用户，则可以使用Calendar。日历有一个方法dateComponents(_:from:to:)，它有以下声明:

func dateComponents(_ components: Set<Calendar.Component>, from start: Date, to end: Date) -> DateComponents

返回两个日期的差值。

下面使用dateComponents(_:from:to:)的Playground代码展示了如何通过只返回一种日历类型的差异来检索两个日期之间的差异。组件(年、月、日、小时或分钟)。

import Foundation

let oldDate = Date(timeIntervalSinceReferenceDate: -16200)
let newDate = Date(timeIntervalSinceReferenceDate: 0)

let descendingOrderedComponents = [Calendar.Component.year, .month, .day, .hour, .minute]
let dateComponents = Calendar.current.dateComponents(Set(descendingOrderedComponents), from: oldDate, to: newDate)
let arrayOfTuples = descendingOrderedComponents.map { ($0, dateComponents.value(for: $0)) }

for (component, value) in arrayOfTuples {
    if let value = value, value > 0 {
        print(component, value) // prints hour 4
        break
    }
}

如果你需要重复这个操作，你可以重构你的代码:

import Foundation

extension Date {
    
    func offset(from: Date) -> (Calendar.Component, Int)? {
        let descendingOrderedComponents = [Calendar.Component.year, .month, .day, .hour, .minute]
        let dateComponents = Calendar.current.dateComponents(Set(descendingOrderedComponents), from: from, to: self)
        let arrayOfTuples = descendingOrderedComponents.map { ($0, dateComponents.value(for: $0)) }
        
        for (component, value) in arrayOfTuples {
            if let value = value, value > 0 {
                return (component, value)
            }
        }
        
        return nil
    }

}

let oldDate = Date(timeIntervalSinceReferenceDate: -16200)
let newDate = Date(timeIntervalSinceReferenceDate: 0)

if let (component, value) = newDate.offset(from: oldDate) {
    print(component, value) // prints hour 4
}

结合扩展+ DateComponentsFormatter从@leo-dabus的答案

Xcode 8.3•Swift 3.1

extension DateComponentsFormatter {
    func difference(from fromDate: Date, to toDate: Date) -> String? {
        self.allowedUnits = [.year,.month,.weekOfMonth,.day]
        self.maximumUnitCount = 1
        self.unitsStyle = .full
        return self.string(from: fromDate, to: toDate)
    }
}

let dateComponentsFormatter = DateComponentsFormatter()
dateComponentsFormatter.difference(from: Date(), to: Date(timeIntervalSinceNow: 4000000)) // "1 month"

Leo Dabus回答的一个小补充，提供复数版本，更易于人类阅读。

斯威夫特3

extension Date {
    /// Returns the amount of years from another date
    func years(from date: Date) -> Int {
        return Calendar.current.dateComponents([.year], from: date, to: self).year ?? 0
    }
    /// Returns the amount of months from another date
    func months(from date: Date) -> Int {
        return Calendar.current.dateComponents([.month], from: date, to: self).month ?? 0
    }
    /// Returns the amount of weeks from another date
    func weeks(from date: Date) -> Int {
        return Calendar.current.dateComponents([.weekOfMonth], from: date, to: self).weekOfMonth ?? 0
    }
    /// Returns the amount of days from another date
    func days(from date: Date) -> Int {
        return Calendar.current.dateComponents([.day], from: date, to: self).day ?? 0
    }
    /// Returns the amount of hours from another date
    func hours(from date: Date) -> Int {
        return Calendar.current.dateComponents([.hour], from: date, to: self).hour ?? 0
    }
    /// Returns the amount of minutes from another date
    func minutes(from date: Date) -> Int {
        return Calendar.current.dateComponents([.minute], from: date, to: self).minute ?? 0
    }
    /// Returns the amount of seconds from another date
    func seconds(from date: Date) -> Int {
        return Calendar.current.dateComponents([.second], from: date, to: self).second ?? 0
    }
    /// Returns the a custom time interval description from another date
    func offset(from date: Date) -> String {
        if years(from: date)   == 1 { return "\(years(from: date)) year"   } else if years(from: date)   > 1 { return "\(years(from: date)) years"   }
        if months(from: date)  == 1 { return "\(months(from: date)) month"  } else if months(from: date)  > 1 { return "\(months(from: date)) month"  }
        if weeks(from: date)   == 1 { return "\(weeks(from: date)) week"   } else if weeks(from: date)   > 1 { return "\(weeks(from: date)) weeks"   }
        if days(from: date)    == 1 { return "\(days(from: date)) day"    } else if days(from: date)    > 1 { return "\(days(from: date)) days"    }
        if hours(from: date)   == 1 { return "\(hours(from: date)) hour"   } else if hours(from: date)   > 1 { return "\(hours(from: date)) hours"   }
        if minutes(from: date) == 1 { return "\(minutes(from: date)) minute" } else if minutes(from: date) > 1 { return "\(minutes(from: date)) minutes" }
        return ""
    }
}

你问:

我想有一个函数，比较两个日期，如果(秒> 60)，那么它返回分钟，如果(分钟> 60)返回小时，如果(小时> 24)返回天，等等。

我假设您正在尝试构建两个日期之间经过时间的字符串表示。而不是自己写代码来做这件事，苹果已经有了一个专门设计来做这件事的类。也就是说，使用DateComponentsFormatter，将allowedUnits设置为对你的应用有意义的任何值，将unitsStyle设置为你想要的任何值(例如.full)，然后调用string(from:to:)。

例如，在Swift 3中:

let previousDate = ...
let now = Date()

let formatter = DateComponentsFormatter()
formatter.unitsStyle = .full
formatter.allowedUnits = [.month, .day, .hour, .minute, .second]
formatter.maximumUnitCount = 2   // often, you don't care about seconds if the elapsed time is in months, so you'll set max unit to whatever is appropriate in your case

let string = formatter.string(from: previousDate, to: now)

这也将本地化对应于相关设备的字符串。

或者，在Swift 2.3中:

let previousDate = ...
let now = NSDate()

let formatter = NSDateComponentsFormatter()
formatter.unitsStyle = .Full
formatter.allowedUnits = [.Month, .Day, .Hour, .Minute, .Second]
formatter.maximumUnitCount = 2

let string = formatter.stringFromDate(previousDate, toDate: now)

如果您正在寻找实际的数值，只需使用dateComponents。例如，在Swift 3中:

let components = Calendar.current.dateComponents([.month, .day, .hour, .minute, .second], from: previousDate, to: now)

或者，在Swift 2.3中:

let components = NSCalendar.currentCalendar().components([.Month, .Day, .Hour, .Minute, .Second], fromDate: previousDate, toDate: now, options: [])

如果有人需要显示所有的时间单位，如“小时分秒”，而不仅仅是“小时”。假设两个日期的时差是1小时59分20秒。该功能将显示“1h 59m 20s”。

下面是我的Objective-C代码:

extension NSDate {

    func offsetFrom(date: NSDate) -> String {

        let dayHourMinuteSecond: NSCalendarUnit = [.Day, .Hour, .Minute, .Second]
        let difference = NSCalendar.currentCalendar().components(dayHourMinuteSecond, fromDate: date, toDate: self, options: [])

        let seconds = "\(difference.second)s"
        let minutes = "\(difference.minute)m" + " " + seconds
        let hours = "\(difference.hour)h" + " " + minutes
        let days = "\(difference.day)d" + " " + hours

        if difference.day    > 0 { return days }
        if difference.hour   > 0 { return hours }
        if difference.minute > 0 { return minutes }
        if difference.second > 0 { return seconds }
        return ""
    }

}

在Swift 3+中:

extension Date {

    func offsetFrom(date: Date) -> String {

        let dayHourMinuteSecond: Set<Calendar.Component> = [.day, .hour, .minute, .second]
        let difference = NSCalendar.current.dateComponents(dayHourMinuteSecond, from: date, to: self)

        let seconds = "\(difference.second ?? 0)s"
        let minutes = "\(difference.minute ?? 0)m" + " " + seconds
        let hours = "\(difference.hour ?? 0)h" + " " + minutes
        let days = "\(difference.day ?? 0)d" + " " + hours

        if let day = difference.day, day          > 0 { return days }
        if let hour = difference.hour, hour       > 0 { return hours }
        if let minute = difference.minute, minute > 0 { return minutes }
        if let second = difference.second, second > 0 { return seconds }
        return ""
    }

}