获取时间戳rtp报文

我使用PacketDotNet从RTP报头检索数据。但有时时间戳为负值。

GetTimeStamp(UdpPacket packetUdp)
{
        byte[] packet = packetUdp.PayloadData;
        long timestamp =  GetRTPHeaderValue(packet, 32, 63);
        return timestamp;
}
private static int GetRTPHeaderValue(byte[] packet, int startBit, int endBit)
    {
        int result = 0;
        // Number of bits in value
        int length = endBit - startBit + 1;
        // Values in RTP header are big endian, so need to do these conversions
        for (int i = startBit; i <= endBit; i++)
        {
            int byteIndex = i / 8;
            int bitShift = 7 - (i % 8);
            result += ((packet[byteIndex] >> bitShift) & 1) *
                      (int)Math.Pow(2, length - i + startBit - 1);
        }
       return result;
    }