Why did we have to push the initialization of currentTime to the end of the init() method ?

struct Current {

    let currentTime : String?
    let temperature : Double
    let precipProbability : Double
    let humidity : Double
    let summary : String
    let icon : String

    init(weatherDictionary: NSDictionary) {

        let currentWeather = weatherDictionary["currently"] as NSDictionary

        self.temperature = currentWeather["temperature"] as Double
        self.precipProbability = currentWeather["precipProbability"] as Double
        self.humidity = currentWeather["humidity"] as Double
        self.summary = currentWeather["summary"] as String
        self.icon = currentWeather["icon"] as String

        let currentTimeIntValue = currentWeather["time"] as Int
        self.currentTime = dateStringFromUnixTime(currentTimeIntValue)

    }

    func dateStringFromUnixTime(unixTime: Int) -> String {
        let timeInSeconds = NSTimeInterval(unixTime)
        let weatherDate = NSDate(timeIntervalSince1970: timeInSeconds)

        let dateFormatter = NSDateFormatter()
        dateFormatter.timeStyle = .ShortStyle

        return dateFormatter.stringFromDate(weatherDate)
    }
}