Doubts with rotation, deltatime and Clamp

Hello everybody.

I am a designer that has recently started to learn scripting with the Unity Scripting Tutorials and I have this problem that gets me totally confused.

I'm trying to do that the canon of a static tank, rotate up and down no more that 90 degrees. However if I use EulerAngles, I get the rotation from 0 to 360 and the Inspector show me between 0 to 180 and from -180 to 0 (and that doesn't help me for understanding whats is going on). Secondly, if I use Time.deltaTime, the Euler Angles goes to a ridiculous value. Finally, if I try to clamp the values the rotation gets crazy.

Below of this text you can see what I am trying to do and 2 pictures for visual help. by the way I haven't wrote the Clamp because the result is totally a mess when I try it.

I couldn't appreciate enough if someone could explain me what I am doing or understanding wrong.

Thank you so much.

Cheers ^w^

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class autoRot : MonoBehaviour {
     private GameObject autoRotCanon;
     public Vector3 canonAngles;
     // Use this for initialization
     void Start () {
         autoRotCanon = GameObject.Find("canon_tank01");
         canonAngles = autoRotCanon.GetComponent<Transform>().eulerAngles;
         Debug.Log(canonAngles);
     }
     
     // Update is called once per frame
     void Update () {
 
         if (Input.GetKey(KeyCode.LeftArrow))
         {
             canonAngles = new Vector3 ((-(canonAngles.x + 10) * Time.deltaTime), 0.0f, 0.0f);
             autoRotCanon.transform.Rotate(canonAngles);
 
         }
 
     }
 }