Swift 3 - UTC to time ago label, thinking 12h / 24h device time changes

问题

I want to convert timeUTC to "5s ago." ago label also thinking 12h / 24h changes on the phone. Because goes crash. Now below lines of code return label value what I want in the Playground. But not on my ios project. It doesn't go into If block. What I'm missing or What's the wrong with that?

    let dateStringUTC = "2016-10-22 12:37:48 +0000"
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss ZZZ"

    if let from = dateFormatter.date(from: dateStringUTC) {

        dateFormatter.dateFormat = "MMM d, yyyy h:mm:ss a"
        let stringFromDate = dateFormatter.string(from: from)
        let dateLast = dateFormatter.date(from: stringFromDate)

        let now = Date()
        let components: NSCalendar.Unit = [.second, .minute, .hour, .day, .weekOfMonth]
        let difference = (Calendar.current as NSCalendar).components(components, from: dateLast!, to: now, options: [])

        if difference.second! <= 0 {
            self.timeInfoLbl.text = "now"
        }
        if difference.second! > 0 && difference.minute! == 0 {
            self.timeInfoLbl.text = String(describing: difference.second!) + "s."
        }
        if difference.minute! > 0 && difference.hour! == 0 {
            self.timeInfoLbl.text = String(describing: difference.minute!) + "m."
        }
        if difference.hour! > 0 && difference.day! == 0 {
            self.timeInfoLbl.text = String(describing: difference.hour!) + "h."
        }
        if difference.day! > 0 && difference.weekOfMonth! == 0 {
            self.timeInfoLbl.text = String(describing: difference.day!) + "d."
        }
        if difference.weekOfMonth! > 0 {
            self.timeInfoLbl.text = String(describing: difference.weekOfMonth!) + "w."

            if difference.weekOfMonth! > 52 {
                let number = Float(difference.weekOfMonth!) / 52
                let strYear = String(format:"%.1f", number)
                self.timeInfoLbl.text = strYear + " year"
            }
        }
    }

回答1:

To convert this string to a date, I would do:

let dateStringUTC = "2016-10-22 12:37:48 +0000"
let dateFormatter = DateFormatter()
dateFormatter.locale = Locale(identifier: "en_US_POSIX")
dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss X"
let date = dateFormatter.date(from: dateStringUTC)!

Then, to show the elapsed time in a nice format, consider DateComponentsFormatter, e.g.:

let now = Date()

let formatter = DateComponentsFormatter()
formatter.unitsStyle = .full
formatter.maximumUnitCount = 2
let string = formatter.string(from: date, to: Date())! + " " + NSLocalizedString("ago", comment: "added after elapsed time to say how long before")

来源：https://stackoverflow.com/questions/41515980/swift-3-utc-to-time-ago-label-thinking-12h-24h-device-time-changes