Problems in c# countdown timer

Hello guys okay?

I'm using the code below to display the counter, the counter starts with 120 seconds and decays. But when I give my scene in the play, the counter begins: 2:59. When he arrives at 2:30, it jumps to 1:29, when he arrives at 1:00, the counter is showing 1:59 and finally when he arrives at 1:30 to 0:29 he jumps and continues to give count normally.

I wonder what is wrong with my script, below:

     using UnityEngine;
     using System.Collections;
     
     public class GameManager : MonoBehaviour {
         
         private float theTimer = 0.0f;
         public float theStartTimer = 120.0f;
         float displayMinutes;
         float displaySeconds;
         
         void Start () {
             theTimer = theStartTimer;
         }
         
         void Update () {
             theTimer -= Time.deltaTime;
             
             if (theTimer <= 0) {
                 Debug.Log("Over Time");
                 theTimer = 0;
             }
             
             if (Input.GetKeyUp(KeyCode.G)) {
                 Debug.Log("Reset time");
                 theTimer = theStartTimer;
                 showRemaining = false;
             }
         }
         
         void OnGUI () {
             displayMinutes = Mathf.CeilToInt(theTimer) / 60.0f;
             displaySeconds = Mathf.CeilToInt(theTimer) % 60.0f;
             string text = string.Format("{0:00}:{1:00}", displayMinutes.ToString("0"), displaySeconds.ToString("0"));
             GUI.Label(new Rect(10, 110, 100, 20), "Time: " + text);
         }
     }
     

Remembering that it compiles and runs normally, only the display of the time you're crazy.