Mathf.Clamp01  

// Set the position of the transform to be that of the time
// but never less than 0 or more than 1
function Update () {
transform.position = Vector3(Mathf.Clamp01(Time.time), 0, 0);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Update() {
transform.position = new Vector3(Mathf.Clamp01(Time.time), 0, 0);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Update():
transform.position = Vector3(Mathf.Clamp01(Time.time), 0, 0)