我最终编写了适合我的应用的代码，以解决这些意外情况，其中TimeZone的缩写格式为"GMT+8"而不是"SGT"。我创建了TimeZone的扩展名：

extension TimeZone {
    static func timeZone(from string: String) -> TimeZone {
        //The string format passed into this function should always be similar to "GMT+8" or "GMT-3:30"

        if string.contains("±") {
            //This case should always be "GMT±00:00",or simply GMT
            return TimeZone(secondsFromGMT: 0)!
        } else {

            //If the string doesn't contain "±",then there should be some offset. We will split the string into timeZone components. "GMT+8" would split into ["GMT","8"]. "GMT-3:30" would split int ["GMT","3","30"]
            let timeZoneComponents = string.components(separatedBy: CharacterSet(charactersIn: "+-:"))
            var isAheadOfGMT: Bool!

            //Check if the string contains "+". This will dictate if we add or subtract seconds from GMT
            if string.contains("+") {
                isAheadOfGMT = true
            } else {
                isAheadOfGMT = false
            }

            //Grab the second element in timeZoneElements. This represents the offset in hours
            let offsetInHours = Int(timeZoneComponents[1])!

            //Convert these hours into seconds
            var offsetInSeconds: Int!
            if isAheadOfGMT {
                offsetInSeconds = offsetInHours * 3600
            } else {
                offsetInSeconds = offsetInHours * -3600
            }

            //Check if there is a colon in the passed string. If it does,then there are additional minutes we need to account for
            if string.contains(":") {
                let additionalMinutes = Int(timeZoneComponents[2])!
                let additionalSeconds = additionalMinutes * 60
                offsetInSeconds += additionalSeconds
            }

            //Create a TimeZone from this calculated offset in seconds
            let timeZoneFromOffset = TimeZone(secondsFromGMT: offsetInSeconds)!

            //Return this value
            return timeZoneFromOffset
        }
    }
}

它的用法如下：

let json: [String:String] = ["timeZone":"GMT+8"]
let timeZone = json["timeZone"]
let birthDate: BirthDate!
if let timeZoneFromAbbrev = TimeZone(abbreviation: timeZone) {
    birthDate = BirthDate(day: birthDay,month: birthMonth,year: birthYear,timeZone: timeZoneFromAbbrev)
} else {       
    let timeZoneFromOffset = TimeZone.timeZone(from: timeZone)
    print(timeZoneFromOffset.abbreviation())
    //Prints "GMT+8"

    birthDate = BirthDate(day: birthDay,timeZone: timeZoneFromOffset)
}

我的BirthDate类上下文：

class BirthDate {
    var day: Int
    var month: Int
    var year: Int
    var timeZone: TimeZone

    init(day: Int,month: Int,year: Int,timeZone: TimeZone) {
        self.day = day
        self.month = month
        self.year = year
        self.timeZone = timeZone
    }
}

时区很有趣。如果有人发现上述TimeZone扩展名有问题，请告诉我。我认为我已经考虑了所有情况，但可能会误会。