来自swift 4的弃用子字符串

Question

import Foundation

class DateHandler {

    private static let stringOffset : Int = 10
    static var todaysDate: String {

        return formatDateString(date: Date())

    }
    static var tomorrowDate: String {
        guard let tommorrowDate = Calendar.current.date(byAdding: .day,value:1, to: Date()) else {return ""}
        return formatDateString(date: tommorrowDate)

    }

    private static func formatDateString(date:Date)->String {

        let dateString = String(describing:date)
        let endIndex = dateString.index(dateString.startIndex, offsetBy:self.stringOffset)
        return dateString.substring(to:endIndex)

    }
}

我在天气应用程序中它给我警告tring切割下标部分范围达到＆＃39;操作