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

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 ""
    }
}

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
}

对Swift 3.0略有修改的代码

let calendar = NSCalendar.current as NSCalendar

// Replace the hour (time) of both dates with 00:00
let date1 = calendar.startOfDay(for: startDateTime)
let date2 = calendar.startOfDay(for: endDateTime)

let flags = NSCalendar.Unit.day
let components = calendar.components(flags, from: date1, to: date2, options: [])

return components.day!

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: [])