String+ISO8601.swift

import Foundation

extension String {
    /// Converts an ISO 8601 formatted `String` into `NSDateComponents`.
    ///
    /// - Note: Does not accept fractional input (e.g.: P3.5Y), must be integers (e.g.: P3Y6M).
    /// - SeeAlso: https://en.wikipedia.org/wiki/ISO_8601#Durations
    /// - Returns: If valid ISO 8601, an `NSDateComponents` representation, otherwise `nil`.
    func ISO8601DateComponents() -> NSDateComponents? {
        // Regex adapted from Moment.js https://github.com/moment/moment/blame/develop/src/lib/duration/create.js#L16
        let pattern = "^P(?:(\\d*)Y)?(?:(\\d*)M)?(?:(\\d*)W)?(?:(\\d*)D)?(?:T(?:(\\d*)H)?(?:(\\d*)M)?(?:(\\d*)S)?)?$"
        let nsstringRepresentation = self as NSString
        let regex = try! NSRegularExpression(pattern: pattern, options: [.anchorsMatchLines])
        
        guard let match = regex.firstMatch(in: self, options: [], range: NSRange(location: 0, length: nsstringRepresentation.length)) else { return nil }
        
        let dateComponents = NSDateComponents()
        
        if match.rangeAt(1).location != NSNotFound, let years = Int(nsstringRepresentation.substring(with: match.rangeAt(1)) as String) {
            dateComponents.year = years
        }
        
        if match.rangeAt(2).location != NSNotFound, let month = Int(nsstringRepresentation.substring(with: match.rangeAt(2)) as String) {
            dateComponents.month = month
        }
        
        if match.rangeAt(3).location != NSNotFound, let week = Int(nsstringRepresentation.substring(with: match.rangeAt(3)) as String) {
            dateComponents.weekOfYear = week
        }
        
        if match.rangeAt(4).location != NSNotFound, let day = Int(nsstringRepresentation.substring(with: match.rangeAt(4)) as String) {
            dateComponents.day = day
        }
        
        if match.rangeAt(5).location != NSNotFound, let hour = Int(nsstringRepresentation.substring(with: match.rangeAt(5)) as String) {
            dateComponents.hour = hour
        }
        
        if match.rangeAt(6).location != NSNotFound, let minute = Int(nsstringRepresentation.substring(with: match.rangeAt(6)) as String) {
            dateComponents.minute = minute
        }
        
        if match.rangeAt(7).location != NSNotFound, let second = Int(nsstringRepresentation.substring(with: match.rangeAt(7)) as String) {
            dateComponents.second = second
        }
        
        return dateComponents
    }
}

Works great for the YouTube API, FYI. It's quite performant.

Note if you're on iOS 10 or higher, Apple added ISO8601DateFormatter, but I needed iOS 9 support.

Hi, does ISO8601DateFormatter support duration? I was not able to use ISO8601DateFormatter to convert duration

I tried and it sadly doesn't seem like it supports duration.