<input type="text" id="input1"/>
<input type="text" id="input2" onchange="tarihayar()"/>
<input type="text" id="input3"/>

	function tarihayar()
	{
		var tarih=document.getElementById('input1').value;
		tarih=tarih.split('-');
		ay=tarih[1]/1;
		yil=tarih[0]/1;
		gun=tarih[2]/1;
		
		var arti=document.getElementById('input2').value;
		ay=ay+(arti/1);
		if (ay>12)
		{
			ay=ay%12;
		}
		
		if (ay<10)
		{
			ay="0"+ay;
		}
		
		document.getElementById('input3').value=yil+'-'+ay+'-'+gun;
		
	}