Time.DeltaTime not correctly incrimenting timer?

So... I may just be stupid, but the simplest part of my little game here seems to be malfunctioning and I do not know why.

In short... In my GameController script (tracks everything) I have

void Start(){ float timer = 0.0f; }

void Update(){ timer += Time.deltaTime; }

Now the game displays the time it took in seconds at the end by simply setting the value of a text field to "timer.ToString();", but it's always way more than the actual time since the scene was loaded.

If my stopwatch times it at, say, 12 seconds - the timer reads over 20. I used my stopwatch to wait exactly 30 seconds, the timer read 120 seconds. I tried to find some kind of consistency to indicate the issue but there doesn't seem to be any... seems to just be fairly randomly "adding" time as it goes along. I've looked into the phenomenon of time drift but I doubt that's my problem.

EDIT: Alrighty, I'll post the full code (At least everything that contains this variable. It's gonna be long lol...

  public GameObject scrollingBackground;
 private GameObject tile1, tile2;
 private float moveSpeed, tileHeight, horizontalMoveSpeed, distanceTraveled;
 public GameObject joystick;
 public float maxShipSpeed, horizontalMovementMultiplier = 1.0f;
 public GameObject ship;
 public GameObject laserPrefab;
 public Text speedText, distanceText, scoreText;
 private float score;
 private float shipHealth, shieldHealth;
 public Image healthBar, shieldBar;
 public float asteroidDamage;
 public float shieldRechargeTimer, shieldRechargeRatePerSecond;
 private float timerForShields;
 private bool shieldIsRecharging = false;
 public GameObject shipDeathExplosion;
 private bool shipAlive = true;
 public GameObject dataTracker;
 -->    private float gameTime;
 private static DataManagerScript dataManager;

 // Use this for initialization
 void Start () {
     dataManager = DataManagerScript.dataManager;
     distanceTraveled = 0;
     tile1 = scrollingBackground.transform.GetChild(0).gameObject;
     tile2 = scrollingBackground.transform.GetChild(1).gameObject;
     tileHeight = tile1.GetComponent<Renderer>().bounds.size.y;
     moveSpeed = 0.0f;
     score = 0;
     shipHealth = 100;
     shieldHealth = 100;
     asteroidDamage = (asteroidDamage == 0) ? 10.0f : asteroidDamage;
     shieldRechargeTimer = (shieldRechargeTimer == 0) ? 5.0f : shieldRechargeTimer;
     shieldRechargeRatePerSecond = (shieldRechargeRatePerSecond == 0) ? 5.0f : shieldRechargeRatePerSecond;
     --> gameTime = 0;
 }
 
 // Update is called once per frame
 void Update () {

     //Timer
     --> gameTime += Time.deltaTime;

     //HEALTH AND SHIELDS
     //SHIELD RECHARGE
     timerForShields += Time.deltaTime;
     if (timerForShields > shieldRechargeTimer)
     {
         timerForShields = 0;
         shieldIsRecharging = true;
     }
     if (shieldHealth < 100)
     {
         shieldHealth += (shieldIsRecharging) ? shieldRechargeRatePerSecond * Time.deltaTime : 0f;
     }
     shieldHealth = (shieldHealth > 100.0f) ? 100 : shieldHealth;

     //SHIELD AND HEALTH BARS
     shieldHealth = (shieldHealth < 0) ? 0.0f : shieldHealth;
     shipHealth = (shipHealth < 0) ? 0 : shipHealth;
     if (shipAlive)
     {
         if (shipHealth <= 0)
         {
             Destroy(ship.GetComponent<MeshRenderer>());
             Instantiate(shipDeathExplosion, ship.transform.position, new Quaternion(0, 0, 0, 0));
             shipAlive = false;
             
             endGameOnDeath();
         }
     }
         shieldBar.rectTransform.localScale = new Vector3(shieldHealth / 100f, 1, 1);
         healthBar.rectTransform.localScale = new Vector3(shipHealth / 100f, 1, 1);
     

     //END HEALTH AND SHIELDS


     //If Joystick Is Down, Increase Movement Speed
     if (joystick.GetComponent<JoystickScript>().isJoystickDown())
     {
         moveSpeed += joystick.GetComponent<JoystickScript>().Vertical() * Time.deltaTime;
         
     }
     //NO GOING BACKWARDS!
     if(moveSpeed < 0)
     {
         moveSpeed = 0;
     }
     // LIMIT MAX SPEED
     moveSpeed = (moveSpeed > maxShipSpeed) ? maxShipSpeed : moveSpeed;

     // End Increase Movement Speed

     //Horizontal Movement
     if (joystick.GetComponent<JoystickScript>().isJoystickDown())
     {
         horizontalMoveSpeed = joystick.GetComponent<JoystickScript>().Horizontal();
         if (horizontalMoveSpeed > 0)
         {
             if (ship.transform.position.x < 7.5)
             {
                 ship.transform.Translate(Vector3.right * horizontalMoveSpeed * horizontalMovementMultiplier * Time.deltaTime);
             }
         }else if(horizontalMoveSpeed < 0)
         {
             if (ship.transform.position.x > -7.5)
             {
                 ship.transform.Translate(Vector3.left * horizontalMoveSpeed * horizontalMovementMultiplier * Time.deltaTime * -1);
             }
         }
       }

     //SCROLLING BACKGROUND
     tile1.transform.Translate(Vector2.down * Time.deltaTime * moveSpeed);
     tile2.transform.Translate(Vector2.down * Time.deltaTime * moveSpeed);
      if(tile1.transform.localPosition.y <= -15)
     {
         
          tile1.transform.localPosition = new Vector3(Random.Range(-10.0f, 10.0f) , tile1.transform.localPosition.y + tileHeight, tile1.transform.localPosition.z);
     }
     if (tile2.transform.localPosition.y <= -15)
     {
         tile2.transform.localPosition = new Vector3(Random.Range(-10.0f, 10.0f), tile2.transform.localPosition.y + tileHeight, tile2.transform.localPosition.z);
     }
     //END SCROLLING BACKGROUND

     distanceTraveled += moveSpeed * Time.deltaTime;

    
     //UI STUFF

     speedText.text = "Speed: " + moveSpeed.ToString("N2");
     distanceText.text = "Distance: " + Mathf.Round(distanceTraveled);
     scoreText.text = score.ToString();
 }

Here is "endGameOnDeath()" that is called when the ship's health is <= 0. PreviousGamesLibrary is just a serializable class that holds the score, distance and time of each game which is subsequently stored in a list of PreviousGamesLibrary's to be saved/loaded.

 private void endGameOnDeath()
 {
     
     dataManager.saveNewGame(new PreviousGamesLibrary(score, distanceTraveled, gameTime));

     Invoke("backToMenu", 1);
 }

As you can see I use quite a bit of Time.DeltaTime, including for a delay in my ship shield's recharging - which I also timed and also works flawlessly.

The only static anything in this entire project is dataManager which is a single-tin structure class for managing persistent data (I'm not going to pretend I know exactly what that means, I followed a tutorial lol) but that's the only thing I have declared static in the entire project.