En el html:
<head>
<style type="text/css">@import url(calendar/calendar-win2k-1.css);</style>
<script type="text/javascript" src="calendar/calendar.js"></script>
<script type="text/javascript" src="calendar/calendar-es.js"></script>
<script type="text/javascript" src="calendar/calendar-setup.js"></script>
</head>
<body>
<input name="purchase_date" id="purchase_date" type="text" class="box2" value="" readonly />
<img src="calendar/calendar.gif" id="btnFecha" style="cursor: pointer;" title="Selecciona fecha" onmouseover="this.style.background='#066190';" onMouseOut="this.style.background=''" />
<script type="text/javascript">fechador('purchase_date')</script>
</body>
Función fechador, para iniciar el sistema del calendario.
function fechador(name, boton){
Calendar.setup({
inputField : name,
ifFormat : "%d/%m/%Y",
button : boton,
//align : "Tl",
singleClick : false,
timeFormat :"24",
showsTime :false,
date : null
});
}

