How to rotate object (z-axis only) slowly? C#

Here you go: Just attach this script to whatever gameobject you want, assign the gameobject you want to rotate under target and set the rotation speed to whatever you want and you should be good to go. I haven't tested this but I'm pretty sure it should work.

 using UnityEngine;
 using System.Collections;
 
 public class RotateOnZAxis : MonoBehaviour {
 
     //Target rotation speed
     public float rotateSpeed = 0.5f;
 
     //Max angle rotation
     public float maxAngle = 30.0f;
 
     //Rotationtarget
     public Transform target;
 
     //Origin Angle
     public float originAngle = 0.0f;
 
     //Reset position?
     public bool resetPosition;
 
     //The Current Angle
     public float currentAngle;
 
     //Actual rotation speed
     private float rotSpeed;
 
     void Update(){
 
         //Calculate target rotation speed
         rotSpeed = rotateSpeed * Time.deltaTime;
 
         //Get the current angle
         currentAngle = target.localEulerAngles.z;
     
         //If current value is between 180 and 360 degrees, get it's negative equivalent
         //This just makes the math easier
         currentAngle = (currentAngle > 180) ? currentAngle - 360 : currentAngle;
 
         //If the current angle is greater than the max angle, set it equal to the max angle
         if (currentAngle > maxAngle) {
             currentAngle = maxAngle;
         }
         //Or If the current angle is smaller than the negative max angle equivalent, 
         //set it equal to the negative max angle equivalen
         else if(currentAngle < -maxAngle){
             currentAngle = -maxAngle;
         }
 
 
         //If press A, rotate the angle to the left, at the specified speed
         if (Input.GetKey (KeyCode.A)){
             if (currentAngle < maxAngle) {
                 target.Rotate (0, 0, rotSpeed);
             }
         }
 
         //If press D, rotate the angle to the right, at the specified rotation speed
         if (Input.GetKey (KeyCode.D)) {
 
         if (currentAngle > -maxAngle) {
                 target.Rotate (0, 0, -rotSpeed);
             
             }
         }
 
         //If either A or D is released, then set reset position to true
         if (Input.GetKeyUp (KeyCode.A) || Input.GetKeyUp (KeyCode.D)) {
             resetPosition = true;
         }
 
         //If reset position is set to true
         if (resetPosition == true) {
         
             //And as long as A or D are not pressed 
             if ((!Input.GetKey (KeyCode.A) && !Input.GetKey (KeyCode.D))) {
 
                 //If the current angle is greater than the origin angle then decrease it's value
                 if (currentAngle > originAngle) {
                     target.Rotate (0, 0, -rotSpeed);
                 } 
                 //Or if the current angle is less than origin angle then increase it's value
                 else if (currentAngle < originAngle) {
                     target.Rotate (0, 0, rotSpeed);
                 }
 
                 //If the current angle is between 0.1 and -0.1 degrees, then set reset to false
                 if (currentAngle < 0.1f && currentAngle > -0.1f) {
                     currentAngle = 0;
                     resetPosition = false;
                 }
             }
         }
     }
 }