In CCacheOutputStream, when writing times int a ccache file, we have
        if (creds.starttime != null)
            write32((int)(creds.starttime.getTime()/1000));
        else
            write32(0);

        if (creds.renewTill != null)
            write32((int)(creds.renewTill.getTime()/1000));
        else
            write32(0);

On the other hand, in CCacheInputStream, we directly read the integer and pass it to new KerberosTime(long). Therefore, null is changed to 1970/1/1 0:0:0.