Directional light not rotating more than 90 degrees.

Ok, so I am using a directional light with a cookie on it to simulate clouds moving across the terrain for a day/night cycle. It works fine, except at noon (when the directional lights x rotation reaches 90) the y and z rotations flip to 180 and the x rotation begins to decrease. This still causes the light to rotate properly, but the cookie then travels backwards. How can I keep it rotating on the x axis to 180 degrees? Or is there a better way to do clouds? Also if anyone knows a good way to switch out the skybox from a day to night, that's the next phase and any advice would be appreciated.

 #pragma strict
 var dayLength:float;
 var curTimeCycle:float;
 var sunUp:float;
 var sunDown:float;
 
 var sun:GameObject;
 var moon:GameObject;
 
 var sunIntensity:float;
 var sunFade:float;
 function Start () 
 {
     print(((sunDown-sunUp)*sunFade));
 }
 
 function Update () 
 {
     sun.transform.rotation.y=0;
     sun.transform.rotation.z=0;
     curTimeCycle+=Time.deltaTime*1;
     //set the sun for sunup
     if((curTimeCycle<sunUp))
     {
         sun.transform.localRotation = Quaternion(0,0,0,0);
     }
     //set the sun for sundown
     else if((curTimeCycle>sunDown))
     {
         sun.transform.localRotation = Quaternion(180,0,0,0);
         sun.light.intensity=0;
     }else{
         sun.transform.Rotate(Vector3((180/(sunDown-sunUp))*Time.deltaTime,0,0), Space.World);
 
         if(curTimeCycle<(sunUp+((sunDown-sunUp)*sunFade)))
         {
             sun.light.intensity+=((sunIntensity/((sunDown-sunUp)*sunFade))*Time.deltaTime);
         }else if(curTimeCycle>(sunDown-((sunDown-sunUp)*sunFade)))
         {
             sun.light.intensity-=((sunIntensity/((sunDown-sunUp)*sunFade))*Time.deltaTime);
         }
     }
     if(curTimeCycle>dayLength)
     {
         curTimeCycle = 0.0;
     }
     
 }