<!DOCTYPE html>

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

<style>

    #div1{width:50px; height:50px; background:red;}

    #div2{width:200px; height:150px; background:grey; display:none;}

</style>

</head>

<body>

    <div id="div1"></div>

    <div id="div2"></div>

    <script>

        window.onload = function(){

            var div1 = document.getElementById("div1");

            var div2 = document.getElementById("div2");

            var timer = null;

            div1.onmouseover = function(){

                clearTimeout(timer);

                timer = setTimeout(function(){

                    div2.style.display = "block";

                },500);

            };

            div1.onmouseout = function(){

                timer = setTimeout(function(){

                    div2.style.display = "none";

                },500);

            };

            div2.onmouseover = function(){

                clearTimeout(timer);

                timer = setTimeout(function(){

                    div2.style.display = "block";

                },500);

            };

            div2.onmouseout = function(){

                timer = setTimeout(function(){

                    div2.style.display = "none";

                },500);

            };

        };

    </script>

</body>

</html>