Código fuente de 'Calcula fecha.php'

<title>Calcula fecha</title>
<body style="font-family: Arial; font-size: 9pt">

<center><b><font face="Arial" size="3">Calcula fecha</font></b>

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if ($submit == 'Calcular') {
	if ($Calcular == 'sub') {
		$day = $day - $days;
	} elseif ($Calcular == 'add') {
		$day = $day + $days;
	$echo = date($output, mktime (0,0,0,$month,$day,$year)); //This is the calculated date

echo "
<form action=\"\" method=post>
<table border=0 cellspacing=1 cellpadding=2 bgcolor=blue width=320>
<td bgcolor=blue><font color=white face=arial><B>Calcula fechas"; if (!empty($echo)) {echo " - Result.: $echo";} echo "</B></font></td>
<td bgcolor=white>
	<table border=0 cellpadding=3 cellspacing=0>
		<td aligin=left><font color=blue face=arial>Elige fecha:  </font></td><td aligin=left><select name=month>";
		getoption(1, 12, date("m"));
		echo "</select><select name=day>";
		getoption(1, 31, date("d"));
		echo "</select><select name=year>";
		getoption(1970, 2020, date("Y"));
		echo "</select></td>
		<td aligin=left><font color=blue face=arial>Suma/resta:  </font></td><td aligin=left><select name=Calcular><option value=add>+<option value=sub>-</select>
	<input type=text name=days size=4 maxlength=4><font color=blue face=arial> día(s)</font></td>
		<td aligin=left><font color=blue face=arial>Formato salida:  </font></td><td aligin=left><select name=output><option value=\"m-d-Y\">mm-dd-YYYY<option value=\"l, M d Y\">Día Semana, mm dd yyyyy</select></td>
		<td aligin=left colspan=2><input type=submit name=submit value=Calcular></td>

function getoption($start, $end, $if) {
	for ($i=$start;$i<=$end;$i++) {
		echo "<option value=$i"; if ($i == $if) { echo " selected";} echo ">$i";