Count-down form a variable

I’m working on this code. I want a count down tool that See his countdown timer time form a variable that is lookup when the counter starts…

How do I build this<<<

/*
* Open serial monitor, press one of the keys below and click 'Send':
* 'S' - Start timer
* 'P' - Pause timer
* 'R' - Restart timer
* 'T' - Stop timer
*/

#include "Countimer.h"

Countimer TIMER;
int CounterStatus = 0;
int CounterStatus1x = 0;

void setup()
{
  Serial.begin(9600);
        

    // HELP >>>> Count-down Need to run form a variable and (re)set later
  TIMER.setCounter( 0, 0, 21, TIMER.COUNT_DOWN, TIMERComplete);
    // Call print_time2() method every 1s.
  TIMER.setInterval(print_time2, 1000);

  Serial.println("Online");

}

void loop()
{

  TIMER.run();

  if (Serial.available() > 0)
  {
    char c = toupper(Serial.read());

    switch (c)
    {
      case 'T':
        TIMER.stop();
        break;
      case 'R':
        TIMER.restart();
        break;
      case 'S':
        TIMER.start();
        CounterStatus = 1;
        CounterStatus1x = 1;
        break;
      case 'P':
        TIMER.pause();
        break;
      default:
        break;
    }
  }

  if (CounterStatus == 0 && CounterStatus1x == 0) {
    Serial.print("OFF \n");
    CounterStatus1x = 1;
  }

}

void print_time2()
{
  Serial.print("Tijd: ");
  Serial.println(TIMER.getCurrentTime());
}


void TIMERComplete()
{
  digitalWrite(13, LOW);
  CounterStatus = 0;
  CounterStatus1x = 0;  
}

You will need to, at a minimum, post a link to the Countimer library. Only you can see what the arguments to setCounter() and setInterval() are.

It would appear that the first 3 arguments to setCounter() define how long the timer is to take to count down. The direction (up or down) appears to be the 4th argument, and the function to call regularly is the 5th argument. The setInterval() call appears to define how often to call the function that is the 5th argument to the setCounter() call.