if($_POST['submit']=='Enter') {
$err = array();
if (isset($_POST['login'])) { $login = $_POST['login'];
if ($login == '') { unset($login);} }
if (isset($_POST['password'])) { $password=$_POST['password'];
if ($password =='') { unset($password);} }
if (empty($login) or empty($password)) {
$err[]='You did not enter all the information you fill in all fields';
}
if (empty($err))
{
$login = stripslashes($login);
$login = htmlspecialchars($login);
$password = stripslashes($password);
$password = htmlspecialchars($password);
$login = trim($login);
$password = trim($password);
$password = md5($password);
// $password = strrev($password);
// $password = $password."b3p6f";
$result1 = mysql_query("SELECT * FROM users WHERE login='".$login."' AND password='".$password."'"); //извлекаем из базы все данные о пользователе с введенным логином
$myrow = mysql_fetch_array($result1);
if (empty($myrow['id'])) {

$err[]='Sorry, you entered your username or password is incorrect.';
} else {
$_SESSION['login']=$myrow['login'];
$_SESSION['password']=$myrow['password'];
$_SESSION['id']=$myrow['id'];
$_SESSION['rememberMe'] = $_POST['rememberMe'];
$_SESSION['dostup'] = $myrow['dostup'];
$_SESSION['business_unit'] = $myrow['business_unit'];
$_SESSION['linemanager'] = $myrow['linemanager'];
$_SESSION['phone_number'] = $myrow['phone_number'];
$_SESSION['lastname'] = $myrow['lastname'];
$_SESSION['firstname'] = $myrow['firstname'];
$_SESSION['post_id'] = $myrow['post_id'];

}
}

Ты хочешь чтобы по id_сессии идентифицировался юзер?
включаем логику и думаем

$result = mysql_query("SELECT `id` FROM `users` WHERE login='$login'");
$myrow = mysql_fetch_array($result);
if (!empty($myrow['id'])) {
$page.='Sorry, you entered username is already registered. Please enter a different username.<br>';
} else {
// если такого нет, то сохраняем данные
$query = "INSERT INTO `users` (lastname,firstname,email,login,password,business_unit,phone_number,post_id,dostup,reminder,doa,dod)
VALUES ('$lastname','$firstname','$email','$login','$password','$business_unit','$phone_number','$post_id','$dostup','$reminder','$doa','$dod')";
if(mysql_query($query))
{ $page.="<p class='soob' style='color:green'>Thanks. Registration is complete.</p>";}
else { $page.='Error! You are not registered.<br>';}
}

<?php
$d=date('Y-m-d [h:i]');
echo "Текущая дата и время на сервере: $d";
?><p>
<label for="doa">Date</label>
<input type="text" name="doa" value="<?=date('y-m-d');?>" id="doa" AUTOCOMPLETE=ON />
<br>^{The date of receipt numbers}
</p>

логично что если юзер зарегался в апреле, то звонков за март у него в теории не должно быть