I just started playing with this script today, and this was the first thing I needed to tackle. I needed the month, day and year in separate boxes, so after some poking around, I was able to modify the Linked fields example to get this to work. Try this:

<input type="text" name="month" id="f_date_a" size="2"/>/
<input type="text" name="day" id="f_date_b" size="2"/>/
<input type="text" name="year" id="f_date_c" size="4"/>
<input type="reset" id="f_trigger_b" value="...">

<script type="text/javascript">
function catcalc(cal) {
var date = cal.date;

var field = document.getElementById("f_date_b");
field.value = date.print("%d");

field = document.getElementById("f_date_c");
field.value = date.print("%Y");
}
Calendar.setup({
inputField : "f_date_a",
ifFormat : "%m",
button : "f_trigger_b",
onUpdate : catcalc
});
Calendar.setup({
inputField : "f_date_b",
ifFormat : "%d"
});
Calendar.setup({
inputField : "f_date_c",
ifFormat : "%Y"
});
</script>