C#

string validstrStart = "validuntil=";
     string validstrEnd = "username=";
     int validstartIndex = result.IndexOf(validstrStart);
     int validendIndex = result.IndexOf(validstrEnd);
     string valid = result.Substring(validstartIndex, validendIndex - validstartIndex);

     double timestampv = Convert.ToDouble(valid);
     System.DateTime dateTime = new System.DateTime(1970, 1, 1, 0, 0, 0, 0);
     dateTime = dateTime.AddSeconds(timestampv);
     string validpro = dateTime.ToShortDateString() + " " + dateTime.ToShortTimeString();

double timestampv = Convert.ToDouble(valid);

uint ToUInt(int value)
{
    return value < 0 ? (uint)(int.MaxValue - value) : (uint)value;
}
int ToInt(uint value)
{
    return value > int.MaxValue ? (int)(int.MaxValue - value) : (int)value;
}