Simple countdown timer for Unity projects that updates a Text (you supply the Text and how many seconds to start at)

Timer.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

[ExecuteInEditMode]
public class CountdownTimer : MonoBehaviour {
  public Text timerText;
  public int startingSeconds;

  private float elapsedTime;

	// Use this for initialization
	void Start () {
		elapsedTime = 0.0f;
    UpdateTimerText();
	}
	
	// Update is called once per frame
	void Update () {
		elapsedTime += Time.deltaTime;
    UpdateTimerText();
	}

  void UpdateTimerText() {
    int timeInSeconds = (int)(elapsedTime % 60);
    int minutes = 0;
    int remainingSeconds = startingSeconds - timeInSeconds;
    if(startingSeconds >= 60) {
      minutes = (int)((startingSeconds - timeInSeconds)/60);
      remainingSeconds -= (minutes * 60);
    }
    timerText.text = minutes.ToString() + ":" + remainingSeconds.ToString().PadLeft(2, '0');
  }
}