Calculate duration between date ios in Years, months and date format

We can use this function in Swift 2.0

func yearsBetweenDate(startDate: NSDate, endDate: NSDate) -> Int
{
    let calendar = NSCalendar.currentCalendar()

    let components = calendar.components([.Year], fromDate: startDate, toDate: endDate, options: [])

    return components.year
}

You can return anything like I returned year in this function. This will return number of years between the two dates.
You can just write months,days etc in order to find the difference between the two dates in months and days respectively.

Edit

Swift 3.0 and Above

func yearsBetweenDate(startDate: Date, endDate: Date) -> Int {

    let calendar = Calendar.current

    let components = calendar.dateComponents([.year], from: startDate, to: endDate)

    return components.year!
}

With Swift 5 and iOS 12, you can use one of the 3 solutions below in order to calculate the difference (in years, months, days) between two dates.

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#1. Using Calendar's dateComponents(_:from:to:) method

Calendar has a method called dateComponents(_:from:to:). dateComponents(_:from:to:) has the following declaration:

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

Returns the difference between two dates specified as DateComponents.

The Playground example below show how to use dateComponents(_:from:to:) in order to compute the difference between two dates:

import Foundation

let calendar = Calendar.current
let startComponents = DateComponents(year: 2010, month: 11, day: 22)
let endComponents = DateComponents(year: 2015, month: 5, day: 1)

let dateComponents = calendar.dateComponents([.year, .month, .day], from: startComponents, to: endComponents)
print(dateComponents) // prints: year: 4 month: 5 day: 9 isLeapMonth: false

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#2. Using Calendar's dateComponents(_:from:to:) method

Calendar has a method called dateComponents(_:from:to:). dateComponents(_:from:to:) has the following declaration:

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

Returns the difference between two dates.

The Playground example below show how to use dateComponents(_:from:to:) in order to compute the difference between two dates:

import Foundation

let calendar = Calendar.current
let startDate = calendar.date(from: DateComponents(year: 2010, month: 11, day: 22))!
let endDate = calendar.date(from: DateComponents(year: 2015, month: 5, day: 1))!

let dateComponents = calendar.dateComponents([.year, .month, .day], from: startDate, to: endDate)
print(dateComponents) // prints: year: 4 month: 5 day: 9 isLeapMonth: false

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#3. Using DateComponentsFormatter's string(from:to:) method

DateComponentsFormatter has a method called string(from:to:). string(from:to:) has the following declaration:

func string(from startDate: Date, to endDate: Date) -> String?

Returns a formatted string based on the time difference between two dates.

The Playground example below show how to use string(from:to:) in order to compute the difference between two dates:

import Foundation

let calendar = Calendar.current
let startDate = calendar.date(from: DateComponents(year: 2010, month: 11, day: 22))!
let endDate = calendar.date(from: DateComponents(year: 2015, month: 5, day: 1))!

let formatter = DateComponentsFormatter()
formatter.unitsStyle = .full
formatter.allowedUnits = [.year, .month, .day]
let string = formatter.string(from: startDate, to: endDate)!
print(string) // prints: 4 years, 5 months, 9 days

If you need the difference (in years, months, days) numerically then compute NSDateComponents as in Swift days between two NSDates or Rajan's answer.

If you need the difference as a (localized) string to present it to the user, then use NSDateComponentsFormatter like this

let form = NSDateComponentsFormatter()
form.maximumUnitCount = 2
form.unitsStyle = .Full
form.allowedUnits = [.Year, .Month, .Day]
let s = form.stringFromDate(date1, toDate: date2)

As already mentioned in the comments, computing the difference from the pure time interval between the dates cannot give correct results because most information about the dates is lost.

Update for Swift 3:

let form = DateComponentsFormatter()
form.maximumUnitCount = 2
form.unitsStyle = .full
form.allowedUnits = [.year, .month, .day]
let s = form.string(from: date1, to: date2)