How Can I make A Platform Rotate 180degrees then Stop and Reset back to 0

I want to make a platform that when a timer hits 0 it rotates 180 degrees and then stops, and then when the timer resets it starts all over again. I've tried using Quaternions but I don't think I know how to use them well enough and or how to use use euler angles with them, help please

 

Just an euler angles rotation on z axis. To 180 and back to 0.

Code (CSharp):

1. public class NPCKilledCounter : MonoBehaviour
2. {
3.     [SerializeField] float _speed = 1;
4.     bool _growingAngle = true;
5.  
6.     private void Update()
7.     {
8.         transform.Rotate((_growingAngle ? Vector3.forward : Vector3.back) * Time.deltaTime * _speed);
9.  
10.         if(_growingAngle && transform.rotation.eulerAngles.x >= 180)
11.         {
12.             _growingAngle = false;
13.             return;
14.         }
15.         if (transform.rotation.eulerAngles.x <= 0) _growingAngle = true;
16.     }
17. }