The use of `var` during initialization

struct DailyWeather {
   // Some properties omitted for clearity
    let sunriseTime: String?
    let sunsetTime: String?
    let day: String?

   // Make our date formatter static, i.e., not instance level
    static let dateFormatter = NSDateFormatter()

    init(dailyWeather: [String: AnyObject]) {
        // Some initialization omitted for clearity
        sunriseTime = DailyWeather.timeStringFromUnixTime(dailyWeather["sunriseTime"] as? Double)
        sunsetTime = DailyWeather.timeStringFromUnixTime(dailyWeather["sunsetTime"] as? Double)
        day = DailyWeather.dayStringFromTime(dailyWeather["time"] as? Double)
    }

    private static func timeStringFromUnixTime(unixTime: Double?) -> String? {
        if unixTime == nil {
            return nil
        }

        let date = NSDate(timeIntervalSince1970: unixTime!)
        dateFormatter.dateFormat = "HH:MM"
        return dateFormatter.stringFromDate(date)
    }

    private static func dayStringFromTime(unixTime: Double?) -> String? {
        if unixTime == nil {
            return nil
        }

        let date = NSDate(timeIntervalSince1970: unixTime!)
        dateFormatter.locale = NSLocale(localeIdentifier: NSLocale.currentLocale().localeIdentifier)
        dateFormatter.dateFormat = "EEEE"
        return dateFormatter.stringFromDate(date)
    }
}