Capture Zone Timer

Hi I'm current creating a script that when the player enters an object a timer in the screen increases. However the timer only goes up by a second or a very little amount every time the player walks in the object. I'm trying to fix it so that once the player enters the object the timer keeps increasing. I know that I have to do something with the update method but I really don't know? I'm kinda new to c# as well! Thanks for all the help!

 public class CapturZone : MonoBehaviour {
     public Text captureText;
 
     public float startTime;
     public string currentTime;
 
     public float targetStartTime;
     public string targetCurrentTime;
     
     // Use this for initialization
     void Start () {
     captureText.text = "Capture Point held for: " + currentTime;
     }
     
     // Update is called once per frame
     void Update () {
 
     }
 
     void OnTriggerEnter(Collider other)
     {
         if(other.gameObject.tag == "Player")
         {
             gameObject.GetComponent<Renderer>().material.color = Color.green;
             startTime += Time.deltaTime;
             currentTime = string.Format("{0:0}", startTime);
             print(currentTime);
             captureText.text = "Capture Point held for: " + currentTime;
             Debug.Log("Player hit capture zone");
         }
         if(other.gameObject.tag == "Target")
         {
             gameObject.GetComponent<Renderer>().material.color = Color.red;
             targetCurrentTime = currentTime;
             targetStartTime -= Time.deltaTime;
             targetCurrentTime = string.Format("{0:0}", startTime);
             print(targetCurrentTime);
             captureText.text = "Capture Point held for: " + targetCurrentTime;
             Debug.Log("Target hit capture zone");
         }
     }
 }
 

Thanks for the help!