如果你的目的是获得两个日期之间的确切天数，你可以这样解决这个问题:

// Assuming that firstDate and secondDate are defined
// ...

var calendar: NSCalendar = NSCalendar.currentCalendar()

// Replace the hour (time) of both dates with 00:00
let date1 = calendar.startOfDayForDate(firstDate)
let date2 = calendar.startOfDayForDate(secondDate)

let flags = NSCalendarUnit.DayCalendarUnit
let components = calendar.components(flags, fromDate: date1, toDate: date2, options: nil)

components.day  // This will return the number of day(s) between dates

  func dateDiff(dateStr:String) -> String {
        var f:NSDateFormatter = NSDateFormatter()
        f.timeZone = NSTimeZone.localTimeZone()
        f.dateFormat = "yyyy-M-dd'T'HH:mm:ss.SSSZZZ"
        
        var now = f.stringFromDate(NSDate())
        var startDate = f.dateFromString(dateStr)
        var endDate = f.dateFromString(now)
        var calendar: NSCalendar = NSCalendar.currentCalendar()
        
        let calendarUnits = NSCalendarUnit.CalendarUnitWeekOfMonth | NSCalendarUnit.CalendarUnitDay | NSCalendarUnit.CalendarUnitHour | NSCalendarUnit.CalendarUnitMinute | NSCalendarUnit.CalendarUnitSecond
        let dateComponents = calendar.components(calendarUnits, fromDate: startDate!, toDate: endDate!, options: nil)
        
        let weeks = abs(dateComponents.weekOfMonth)
        let days = abs(dateComponents.day)
        let hours = abs(dateComponents.hour)
        let min = abs(dateComponents.minute)
        let sec = abs(dateComponents.second)
        
        var timeAgo = ""

        if (sec > 0){
            if (sec > 1) {
                timeAgo = "\(sec) Seconds Ago"
            } else {
                timeAgo = "\(sec) Second Ago"
            }
        }
        
        if (min > 0){
            if (min > 1) {
                timeAgo = "\(min) Minutes Ago"
            } else {
                timeAgo = "\(min) Minute Ago"
            }
        }
        
        if(hours > 0){
            if (hours > 1) {
                timeAgo = "\(hours) Hours Ago"
            } else {
                timeAgo = "\(hours) Hour Ago"
            }
        }
        
        if (days > 0) {
            if (days > 1) {
                timeAgo = "\(days) Days Ago"
            } else {
                timeAgo = "\(days) Day Ago"
            }
        }
        
        if(weeks > 0){
            if (weeks > 1) {
                timeAgo = "\(weeks) Weeks Ago"
            } else {
                timeAgo = "\(weeks) Week Ago"
            }
        }
        
        print("timeAgo is===> \(timeAgo)")
        return timeAgo;
    }

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
}

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 ""
    }

}

如果你的目的是获得两个日期之间的确切天数，你可以这样解决这个问题:

// Assuming that firstDate and secondDate are defined
// ...

var calendar: NSCalendar = NSCalendar.currentCalendar()

// Replace the hour (time) of both dates with 00:00
let date1 = calendar.startOfDayForDate(firstDate)
let date2 = calendar.startOfDayForDate(secondDate)

let flags = NSCalendarUnit.DayCalendarUnit
let components = calendar.components(flags, fromDate: date1, toDate: date2, options: nil)

components.day  // This will return the number of day(s) between dates

我在Leo Dabus的asnwer中添加了一个“长”版本，以防你想要一个字符串，说“2周前”而不是“2w”……

extension Date {
    func offsetLong(from date: Date) -> String {
        if years(from: date) > 0 {
            return years(from: date) > 1 ? "\(years(from: date)) years ago" : "\(years(from: date)) year ago"
        }
        if months(from: date) > 0 {
            return months(from: date) > 1 ? "\(months(from: date)) months ago" : "\(months(from: date)) month ago" 
        }
        if weeks(from: date) > 0 {
            return weeks(from: date) > 1 ? "\(weeks(from: date)) weeks ago" : "\(weeks(from: date)) week ago"
        }
        if days(from: date) > 0 {
            return days(from: date) > 1 ? "\(days(from: date)) days ago" : "\(days(from: date)) day ago" 
        }
        if hours(from: date) > 0 {
            return hours(from: date) > 1 ? "\(hours(from: date)) hours ago" : "\(hours(from: date)) hour ago"
        }
        if minutes(from: date) > 0 {
            return minutes(from: date) > 1 ? "\(minutes(from: date)) minutes ago" : "\(minutes(from: date)) minute ago"
        }
        if seconds(from: date) > 0 {
            return seconds(from: date) > 1 ? "\(seconds(from: date)) seconds ago" : "\(seconds(from: date)) second ago"
        }
        return ""
    }
}