Something

function update_date(obj){
	
		  var poststr = "p_id=" + encodeURI( document.getElementById("p_id").value ) +
		  			"&month=" + encodeURI( document.getElementById("month").value ) +
					"&day=" + encodeURI( document.getElementById("day").value ) +
					"&dealer_date=" + encodeURI( document.getElementById("dealer_date").value ) +
				    "&year=" + encodeURI( document.getElementById("year").value );  
					
					
    	 request('posts.php', 'the_date', 'POST', poststr);
		
	
	}


<input type="checkbox" value="1" id="dealer_date" />


<span onclick="update_date(document.getElementById('myform'));">
		<img src="../icons/ok.png" />
		</span>