Problem with timer in Update function

I have a script which assigns a random number at the beginning of the scene, I have a few if statements to assign a value in seconds to the timer depending on what the number is. Here is some code:

 using UnityEngine;
 using System.Collections;
 
 public class numberOfTaps : MonoBehaviour 
 {
     private float timer;
     private bool ranOnce = false;
 
     private int tapCounter = 0;
     private int number = 0;
 
     void Start()
     {
             //Generate a random number
         number = Random.Range (50, 250);
     }
 
     void Update () 
     {
             
         randomTapping();
 
     }
 
     void randomTapping()
     {
 
         Debug.Log (number);
 
         if(Input.GetButtonDown ("Fire1"))
         {
             tapCounter += 1;
         }

             //Checks to see if the tap count matches the number and turns the object red
         if(tapCounter == number)
         {
             renderer.material.color = Color.red;
         }
 
         //Debug.Log (tapCounter);


         //Sets the timer according to the number given
         if(number >= 50 && number <= 75)
         {
             timer = 10.0f;
         }
         
         else if(number >= 76 && number <= 125)
         {
             timer = 20.0f;
         }
         
         else if(number >= 126 && number <= 175)
         {
             timer = 30.0f;
         }
         
         else if(number >= 176 && number <= 250)
         {
             timer = 45.0f;
         }
         
         //Count down the timer in seconds
         timer -= Time.deltaTime;
         Debug.Log(timer);

         //Destroy the object if the timer reaches zero
         if(timer <= 0)
         {
             Destroy(gameObject);
         }
 
     }

 //Print out relevant information on screen
     void OnGUI()
     {
         GUI.Label(new Rect(10, 10, 100, 20), "Number of taps: " + tapCounter);
         GUI.Label (new Rect (10, 30, 100, 20), "RandomNumber: " + number);
         GUI.Label(new Rect(10, 50, 100, 20), "Time left: " + timer);
 
     }
 
 }

This is in the update function, i can get my random number alright but the timer never counts down as it keeps restarting, is there a way i can get timer to count to zero before it reaches the end of the code block?