there is no point/need to use hidden form elements:
<?
if (!isset($_POST['submit']))
{
echo '
<form action="" method="POST">
start date: <input type="text" name="startdate"><br>
start time:<input type="text" name="starttime"><br>
end date: <input type="text" name="enddate"><br>
end time: <input type="text" name="endtime"><br>
<input type="submit" name="submit" value="submit">
</form>
';
}
else
{
mysql_connect('host', 'username', 'password');
mysql_select_db('database');
$result = mysql_query("INSERT INTO table SET
startdatetime = '" . $_POST['startdate'] . ' ' . $_POST['starttime'] . "',
enddatetime = '" . $_POST['enddate'] . ' ' . $_POST['endtime'] . "'
");
if ($result) {echo 'INSERT OK';}
else {echo mysql_error();}
}
?>