C# string類型的毫秒轉換為DateTime類型的時間

public DateTime StringToDateTime(string createTime)
{
    double timeVal;
    if (!string.IsNullOrEmpty(createTime))
    {
        timeVal = double.Parse(createTime);
    
        return DateTime.Parse(DateTime.Now.ToString("1970-01-01       00:00:00")).AddMilliseconds(timeVal);;
    }          
}