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

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"

斯威夫特5

func dateDiff(dateStr:String) -> String {
    let f:DateFormatter = DateFormatter()
    f.timeZone = NSTimeZone.local
      f.dateFormat = "yyyy-M-dd'T'HH:mm:ss.SSSZZZ"
      
    let now = f.string(from: NSDate() as Date)
    let startDate = f.date(from: dateStr)
    let endDate = f.date(from: now)
    var _: NSCalendar = NSCalendar.current as NSCalendar
      
    
    let dateComponents = Calendar.current.dateComponents([ .weekOfMonth, .day , .hour , .minute , .second], from: startDate!, to: endDate!)
      
      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;
  }

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

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 ""
    }
}

import Foundation

extension DateComponents {

    func dateComponentsToTimeString() -> String {

        var hour = "\(self.hour!)"
        var minute = "\(self.minute!)"
        var second = "\(self.second!)"

        if self.hour! < 10 { hour = "0" + hour }
        if self.minute! < 10 { minute = "0" + minute }
        if self.second! < 10 { second = "0" + second }

        let str = "\(hour):\(minute):\(second)"
        return str
    }

}

extension Date {

    func offset(from date: Date)-> DateComponents {
        let components = Set<Calendar.Component>([.second, .minute, .hour, .day, .month, .year])
        let differenceOfDate = Calendar.current.dateComponents(components, from: date, to: self)
        return differenceOfDate
    }
}

Use:

var durationString: String {
        return self.endTime.offset(from: self.startTime).dateComponentsToTimeString()
    }