#include a_samp

// Declare a GLOBAL variable to store the current countdown value in
new g_countdownSeconds;

// Declare a GLOBAL variable to store the timer ID in
new g_TIMER_countdown = -1;

stock StartCountdown(seconds)
{
    g_countdownSeconds = seconds;

    SetTimer("TIMER_CountdownStep", 1000, true);

    // If using a textdraw, create it here, BEFORE UpdateCountdownDisplay.

    UpdateCountdownDisplay();
    return 1;
}

forward TIMER_CountdownStep();
public TIMER_CountdownStep()
{
    g_countdownSeconds--;
    UpdateCountdownDisplay();

    if(g_countdownSeconds == 0) KillTimer(g_TIMER_countdown);
    return 1;
}

stock UpdateCountdownDisplay()
{
    // use GameTextForAll/Player or (Player)TextDrawUpdate to update anything
    // that is displaying the countdown. The value of g_countdownSeconds is
    // the time left IN SECONDS. Use format to show this.

    new cdstring[4];
    valstr(cdstring, g_countdownSeconds); // You can use format instead.
    GameTextForAll(cdstring, 2000, 3);
}