float deltaTime = 0;
unsigned long previousMillis = 0;

float timerA = 0;
float sendDataTimer = 0;
bool awaitingResponse = false;

loop()
{
    //First we calculate deltaTime, which is what we'll use from now on (Look down below for the UpdateDeltaTime() code)

    UpdateDeltaTime();


    // Now, say you want something to happen every 20 seconds, like sending data.

    sendDataTimer += deltaTime // every second, at 1.0 to it

    if(sendDataTimer >= 20) // can replace 20 with sendDataInterval for easier editing later
    {
        //do something every 20 seconds, like
        SendData(); //runs the SentData function below
        sendDataTimer = 0; //reset the "every 20 seconds" timer, so it can start counting up to 20 again
    }


    // Are we awaiting a response? If so, add deltaTime to the timeouttimer.
    if(awaitingResponse == true)
    {
        timeoutTimer += deltaTime;
        if(timeoutTimer >= 3) //can replace 3 with timeoutPatience or something for easier editing later
        {
            //we didn't get a response! Bugger.
            awaitingResponse = false;
            //do whatever you want to do when you waited too long for a reponse
        }
    }

}

UpdateDeltaTime()
{
    unsigned long currentMillis = millis();
    deltaTime = currentMillis - previousMillis ;
    Serial.print(deltaTime); // deltaTime is how long the last loop() cycle took
    previousMillis = currentMillis;

    deltaTime *= 0.001;

    if(deltaTime < 0) //this will happen when millis() resets
    {
        // you can do a few things here, you can just set deltaTime to 0, or remember what the last good deltaTime is
        deltaTime = 0; // good enough
    }

}

SendData()
{
    //send that data, and also resets the timeoutTimer;
    timeoutTimer = 0;
    awaitingResponse = true; // this allows the timeoutTimer to run
}