Non-synchronized nodes in different machines. What to do?

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I have two different machines connected by wifi sharing a ROS_MASTER. One of them is the erratic robot which is sending odom messages continuously. The point is that those messages are being received in the other machine with header.stamp > ros::Time::now() ... when it should be header.stamp <= ros::Time::now(). The time distance is ~0.001s. The machines's clocks clearly aren't synchronized (a past reading can never came from the future).

I don't know in depth how NTP works but I expected it would be working properly. What should I do? Is there any typical way to afford this problem?

In the case of the erratic robot I've been diving a bit in its driver and we get that the stamp comes from the internal player code:

   player_msghdr_t* hdr = msg->GetHeader();

        if ((hdr->type == PLAYER_MSGTYPE_DATA) &&
            (hdr->subtype == PLAYER_POSITION2D_DATA_STATE) &&
            (hdr->addr.interf == PLAYER_POSITION2D_CODE))
        {
            // Cast the message payload appropriately
            player_position2d_data_t* pdata = (player_position2d_data_t*) msg->GetPayload();

[...]

odom.header.frame_id = tf::resolve(tf_prefix_, odom_frame_id);
            odom.header.stamp.sec = (long long unsigned int) floor(hdr->timestamp);
            odom.header.stamp.nsec = (long long unsigned int) ((hdr->timestamp - floor(hdr->timestamp)) * 1000000000ULL);

Regards.