I cannot seem to get datediff to work in my simple function called by pressing a button.

It does not return any value for var day_calc.

function date(form) {

d=eval(form.today_date.value);
e=eval(form.closing_date.value);
f=eval(form.day_calc.value);

form.today_date.value='31-Jan-09';
form.closing_date.value='31-Jan-10';

form.day_calc.value = DateDiff("d", '31-Jan-09', '31-Jan-09');
}