diff --git a/htdocs/admin/security_other.php b/htdocs/admin/security_other.php
index f0ad4352b60..f642cd64182 100644
--- a/htdocs/admin/security_other.php
+++ b/htdocs/admin/security_other.php
@@ -177,7 +177,11 @@ $sessiontimeout = ini_get("session.gc_maxlifetime");
 if (empty($conf->global->MAIN_SESSION_TIMEOUT)) $conf->global->MAIN_SESSION_TIMEOUT = $sessiontimeout;
 print '<tr class="oddeven">';
 print '<td>'.$langs->trans("SessionTimeOut").'</td><td class="right">';
-print $form->textwithpicto('', $langs->trans("SessionExplanation", ini_get("session.gc_probability"), ini_get("session.gc_divisor")));
+if (ini_get("session.gc_probability") == 0) {
+	print $form->textwithpicto('', $langs->trans("SessionsPurgedByExternalSystem", ini_get("session.gc_maxlifetime")));
+} else {
+	print $form->textwithpicto('', $langs->trans("SessionExplanation", ini_get("session.gc_probability"), ini_get("session.gc_divisor"), ini_get("session.gc_maxlifetime")));
+}
 print '</td>';
 print '<td class="nowrap">';
 print '<input class="flat" name="MAIN_SESSION_TIMEOUT" type="text" size="6" value="'.htmlentities($conf->global->MAIN_SESSION_TIMEOUT).'"> '.strtolower($langs->trans("Seconds"));
@@ -185,7 +189,6 @@ print '</td>';
 print '</tr>';
 
 
-$sessiontimeout = ini_get("session.gc_maxlifetime");
 if (empty($conf->global->MAIN_APPLICATION_TITLE)) $conf->global->MAIN_APPLICATION_TITLE = "";
 print '<tr class="oddeven">';
 print '<td>'.$langs->trans("MAIN_APPLICATION_TITLE").'</td><td class="right">';
diff --git a/htdocs/langs/en_US/admin.lang b/htdocs/langs/en_US/admin.lang
index 5e485c86dbc..84c64a23615 100644
--- a/htdocs/langs/en_US/admin.lang
+++ b/htdocs/langs/en_US/admin.lang
@@ -1145,6 +1145,7 @@ AvailableModules=Available app/modules
 ToActivateModule=To activate modules, go on setup Area (Home->Setup->Modules).
 SessionTimeOut=Time out for session
 SessionExplanation=This number guarantees that the session will never expire before this delay, if the session cleaner is done by Internal PHP session cleaner (and nothing else). Internal PHP session cleaner does not guarantee that the session will expire after this delay. It will expire, after this delay, and when the session cleaner is run, so every <b>%s/%s</b> access, but only during access made by other sessions (if value is 0, it means clearing of session is done only by an external process).<br>Note: on some servers with an external session cleaning mechanism (cron under debian, ubuntu ...), the sessions can be destroyed after a period defined by an external setup, no matter what the value entered here is.
+SessionsPurgedByExternalSystem=Sessions on this server seems to be cleaned by an external mechanism (cron under debian, ubuntu ...), probably every <b>%s</b> seconds (= value of parameter <b>session.gc_maxlifetime</b>), so changing the value here has no effect. You must ask the server administrator to change session delay.
 TriggersAvailable=Available triggers
 TriggersDesc=Triggers are files that will modify the behavior of Dolibarr workflow once copied into the directory <b>htdocs/core/triggers</b>. They realize new actions, activated on Dolibarr events (new company creation, invoice validation, ...).
 TriggerDisabledByName=Triggers in this file are disabled by the <b>-NORUN</b> suffix in their name.