diff --git a/lang/linuxfoo-gitlab-de_DE.po b/lang/linuxfoo-gitlab-de_DE.po
index ba1dc7b..b696844 100644
--- a/lang/linuxfoo-gitlab-de_DE.po
+++ b/lang/linuxfoo-gitlab-de_DE.po
@@ -2,15 +2,14 @@
 # This file is distributed under the same license as the Gitlab REST API Shortcodes plugin.
 msgid ""
 msgstr ""
-"Project-Id-Version: Gitlab REST API Shortcodes 1.1\n"
-"Report-Msgid-Bugs-To: https://tk-sls.de/ref/linuxfoo-gitlab\n"
+"Project-Id-Version: Gitlab REST API Shortcodes 1.2\n"
+"Report-Msgid-Bugs-To: https://wordpress.org/support/plugin/linuxfoo-gitlab\n"
 "Last-Translator: Tilman Kranz <t.kranz@tk-sls.de>\n"
-"Language: de\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"
-"POT-Creation-Date: 2021-11-27T03:05:12+01:00\n"
-"PO-Revision-Date: 2021-11-27 00:00+0100\n"
+"POT-Creation-Date: 2021-11-27T11:51:10+01:00\n"
+"PO-Revision-Date: 2021-11-27T11:51:10+01:00\n"
 "X-Generator: WP-CLI 2.5.0\n"
 "X-Domain: linuxfoo-gitlab\n"
 
@@ -18,35 +17,82 @@ msgstr ""
 msgid "Gitlab REST API Shortcodes"
 msgstr "Gitlab REST API Shortcodes"
 
+#. Plugin URI of the plugin
+msgid "http://tk-sls.de/ref/gitlab-list-commits"
+msgstr "http://tk-sls.de/ref/gitlab-list-commits"
+
 #. Description of the plugin
-msgid "<p>Summary: Embed list of most recent commits to a Gitlab project from a Gitlab instance's public REST API.</p><p>Example: Add [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since=\"3 month\" max=5] for a list of at most 5 commits to that project that were made since at most 3 months ago.</p><p>The attributes \"url\" and \"project_id\" are mandatory.</p><p>The attributes \"since\" and \"max\" are optional (beware, the default is an unlimited list of commits).</p><p>Other optional attributes:</p><ul><li>ref_name=STRING: List only commit from branch or reference name STRING.</li><li>default_branch: If set, list only commits from the project's default branch.</li></ul>"
-msgstr "<p>Zusammenfassung: Lies Liste neuester Commits eines Projekts aus einem Projekt in einer Gitlab-Instanz via v4 REST API.</p><p>Beispiel: [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since=\"3 month\" max=5] Liste bis zu 5 Commits (Neueste zuerst) aus den letzten 3 Monaten.</p><p>Attribute \"url\" und \"project_id\" sind Pflichtangaben.</p><p>Attribute \"since\" und \"max\" sind optional (Achtung, Standardverhalten ist unlimitiertes Auflisten).</p><p>Weitere optionale Attribute:</p><ul><li>ref_name=STRING: Zeige nur Commits aus Branch oder Refname STRING.</li><li>default_branch: Falls gesetzt, zeige nur Commits aus dem Default-Branch des Projektes.</li></ul>"
+msgid "Summary: Embed list of most recent commits to a Gitlab project from a Gitlab instance's public REST API. Example: Add [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since=\"3 month\" max=5] for a list of at most 5 commits to that project that were made since at most 3 months ago."
+msgstr "Zusammenfassung: Bette Liste der neuesten Commits eines Gitlab-Projektes ein, die mit Gitlabs REST API abgerufen wird. Beispiel: Füge folgenden Shortcode zu einem Artikel hinzu, um die letzten 5 Commits anzuzeigen, die nicht älter als 3 Monate sind: [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since=\"3 month\" max=5]"
 
-#: linuxfoo-gitlab.php:36
+#. Author of the plugin
+msgid "Tilman Kranz"
+msgstr "Tilman Kranz"
+
+#. Author URI of the plugin
+msgid "https://tk-sls.de"
+msgstr "https://tk-sls.de"
+
+#: linuxfoo-gitlab.php:39
+msgid "days"
+msgstr "Tagen"
+
+#: linuxfoo-gitlab.php:42
+msgid "months"
+msgstr "Monaten"
+
+#: linuxfoo-gitlab.php:45
+msgid "years"
+msgstr "Jahren"
+
+#. translators: %d: count, %s unit of time interval
+#: linuxfoo-gitlab.php:49
+msgid "since %d %s ago"
+msgstr "seit %d %s"
+
+#: linuxfoo-gitlab.php:59
 msgid "Required parameter missing."
-msgstr "Benötigter Parameter fehlt."
+msgstr "Notwendiger Parameter fehlt"
 
-#: linuxfoo-gitlab.php:43
+#: linuxfoo-gitlab.php:62
+#: linuxfoo-gitlab.php:68
+msgid "Invalid value for parameter \"project_id\"."
+msgstr "Unzulässiger Wert für Parameter \"project_id\"."
+
+#: linuxfoo-gitlab.php:65
+msgid "Invalid value for parameter \"url\"."
+msgstr "Unzulässiger Wert für Parameter \"url\"."
+
+#: linuxfoo-gitlab.php:75
 msgid "Project URL not reachable."
-msgstr "Projekt-URL nicht erreichbar."
+msgstr "Gitlab-Projekt-URL nicht erreichbar."
 
-#: linuxfoo-gitlab.php:79
+#: linuxfoo-gitlab.php:86
+msgid "Gitlab project"
+msgstr "Gitlab-Projekt"
+
+#: linuxfoo-gitlab.php:93
+msgid "Invalid value for parameter \"since\"."
+msgstr "Unzulässiger Wert für Parameter \"since\"."
+
+#: linuxfoo-gitlab.php:115
 msgid "Commits URL not reachable."
-msgstr "Commits-URL nicht erreichbar."
+msgstr "Gitlab-Commits-URL nicht erreichbar."
 
-#: linuxfoo-gitlab.php:85
+#: linuxfoo-gitlab.php:121
 msgid "Commits information not readable."
-msgstr "Commit-Information nicht verfügbar."
+msgstr "Commit-Information nicht lesbar."
 
-#: linuxfoo-gitlab.php:96
+#: linuxfoo-gitlab.php:132
 msgid "No commits"
-msgstr "Keine Commits"
+msgstr "Keine aktuellen Commits"
 
-#: linuxfoo-gitlab.php:97
-#: linuxfoo-gitlab.php:110
+#: linuxfoo-gitlab.php:133
+#: linuxfoo-gitlab.php:147
 msgid "in branch"
-msgstr "in Branch"
+msgstr "im Branch"
 
-#: linuxfoo-gitlab.php:109
+#. translators: %d: maximum number of commits displayed
+#: linuxfoo-gitlab.php:146
 msgid "Last %d commits"
 msgstr "Letzte %d Commits"
diff --git a/lang/linuxfoo-gitlab.pot b/lang/linuxfoo-gitlab.pot
index fb32698..d37e670 100644
--- a/lang/linuxfoo-gitlab.pot
+++ b/lang/linuxfoo-gitlab.pot
@@ -2,14 +2,15 @@
 # This file is distributed under the same license as the Gitlab REST API Shortcodes plugin.
 msgid ""
 msgstr ""
-"Project-Id-Version: Gitlab REST API Shortcodes 1.1\n"
+"Project-Id-Version: Gitlab REST API Shortcodes 1.2\n"
 "Report-Msgid-Bugs-To: https://wordpress.org/support/plugin/linuxfoo-gitlab\n"
-"Last-Translator: Tilman Kranz <t.kranz@tk-sls.de>\n"
+"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
+"Language-Team: LANGUAGE <LL@li.org>\n"
 "MIME-Version: 1.0\n"
 "Content-Type: text/plain; charset=UTF-8\n"
 "Content-Transfer-Encoding: 8bit\n"
-"POT-Creation-Date: 2021-11-27T03:05:12+01:00\n"
-"PO-Revision-Date: 2021-11-27 00:00+0100\n"
+"POT-Creation-Date: 2021-11-27T12:01:58+01:00\n"
+"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "X-Generator: WP-CLI 2.5.0\n"
 "X-Domain: linuxfoo-gitlab\n"
 
@@ -17,35 +18,82 @@ msgstr ""
 msgid "Gitlab REST API Shortcodes"
 msgstr ""
 
-#. Description of the plugin
-msgid "<p>Summary: Embed list of most recent commits to a Gitlab project from a Gitlab instance's public REST API.</p><p>Example: Add [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since=\"3 month\" max=5] for a list of at most 5 commits to that project that were made since at most 3 months ago.</p><p>The attributes \"url\" and \"project_id\" are mandatory.</p><p>The attributes \"since\" and \"max\" are optional (beware, the default is an unlimited list of commits).</p><p>Other optional attributes:</p><ul><li>ref_name=STRING: List only commit from branch or reference name STRING.</li><li>default_branch: If set, list only commits from the project's default branch.</li></ul>"
+#. Plugin URI of the plugin
+msgid "http://tk-sls.de/ref/gitlab-list-commits"
 msgstr ""
 
-#: linuxfoo-gitlab.php:36
+#. Description of the plugin
+msgid "Summary: Embed list of most recent commits to a Gitlab project from a Gitlab instance's public REST API. Example: Add [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since=\"3 month\" max=5] for a list of at most 5 commits to that project that were made since at most 3 months ago."
+msgstr ""
+
+#. Author of the plugin
+msgid "Tilman Kranz"
+msgstr ""
+
+#. Author URI of the plugin
+msgid "https://tk-sls.de"
+msgstr ""
+
+#: linuxfoo-gitlab.php:39
+msgid "days"
+msgstr ""
+
+#: linuxfoo-gitlab.php:42
+msgid "months"
+msgstr ""
+
+#: linuxfoo-gitlab.php:45
+msgid "years"
+msgstr ""
+
+#. translators: %d: count, %s unit of time interval
+#: linuxfoo-gitlab.php:49
+msgid "since %d %s ago"
+msgstr ""
+
+#: linuxfoo-gitlab.php:59
 msgid "Required parameter missing."
 msgstr ""
 
-#: linuxfoo-gitlab.php:43
+#: linuxfoo-gitlab.php:62
+#: linuxfoo-gitlab.php:68
+msgid "Invalid value for parameter \"project_id\"."
+msgstr ""
+
+#: linuxfoo-gitlab.php:65
+msgid "Invalid value for parameter \"url\"."
+msgstr ""
+
+#: linuxfoo-gitlab.php:75
 msgid "Project URL not reachable."
 msgstr ""
 
-#: linuxfoo-gitlab.php:79
+#: linuxfoo-gitlab.php:86
+msgid "Gitlab project"
+msgstr ""
+
+#: linuxfoo-gitlab.php:93
+msgid "Invalid value for parameter \"since\"."
+msgstr ""
+
+#: linuxfoo-gitlab.php:115
 msgid "Commits URL not reachable."
 msgstr ""
 
-#: linuxfoo-gitlab.php:85
+#: linuxfoo-gitlab.php:121
 msgid "Commits information not readable."
 msgstr ""
 
-#: linuxfoo-gitlab.php:96
+#: linuxfoo-gitlab.php:132
 msgid "No commits"
 msgstr ""
 
-#: linuxfoo-gitlab.php:97
-#: linuxfoo-gitlab.php:110
+#: linuxfoo-gitlab.php:133
+#: linuxfoo-gitlab.php:147
 msgid "in branch"
 msgstr ""
 
-#: linuxfoo-gitlab.php:109
+#. translators: %d: maximum number of commits displayed
+#: linuxfoo-gitlab.php:146
 msgid "Last %d commits"
 msgstr ""
diff --git a/linuxfoo-gitlab.php b/linuxfoo-gitlab.php
index 174338c..50bdab6 100644
--- a/linuxfoo-gitlab.php
+++ b/linuxfoo-gitlab.php
@@ -3,9 +3,9 @@
 /*
 Plugin Name: Gitlab REST API Shortcodes
 Plugin URI: http://tk-sls.de/ref/gitlab-list-commits
-Description: <p>Summary: Embed list of most recent commits to a Gitlab project from a Gitlab instance's public REST API.</p><p>Example: Add [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since="3 month" max=5] for a list of at most 5 commits to that project that were made since at most 3 months ago.</p><p>The attributes "url" and "project_id" are mandatory.</p><p>The attributes "since" and "max" are optional (beware, the default is an unlimited list of commits).</p><p>Other optional attributes:</p><ul><li>ref_name=STRING: List only commit from branch or reference name STRING.</li><li>default_branch: If set, list only commits from the project's default branch.</li></ul>
+Description: Summary: Embed list of most recent commits to a Gitlab project from a Gitlab instance's public REST API. Example: Add [gitlab-list-commits url=https://tk-sls.de/gitlab project_id=42 since="3 month" max=5] for a list of at most 5 commits to that project that were made since at most 3 months ago.
 Author: Tilman Kranz
-Version: 1.1
+Version: 1.2
 Author URI: https://tk-sls.de
 */
 
@@ -13,8 +13,12 @@ class LinuxfooGitlab {
 	public function __construct() {
 	}
 
+    static function load_textdomain() {
+        load_plugin_textdomain('linuxfoo-gitlab', false, dirname(plugin_basename(__FILE__)).'/lang');
+    }
+
     static function css() {
-	    wp_enqueue_style('linuxfoo_gitlab_css', plugins_url('styles.css', __FILE__ ), '', '1.1' );
+	    wp_enqueue_style('linuxfoo_gitlab_css', plugins_url('styles.css', __FILE__ ), '', '1.2' );
     }
 
 	static function error ( $msg ) {
@@ -26,6 +30,25 @@ class LinuxfooGitlab {
             '</div>';
     }
 
+    static function format_since($since) {
+        preg_match('/^([0-9]+)\s+(.*)$/', $since, $m);
+        $num = $m[1];
+        $unit = $m[2];
+
+        if($unit=='days') {
+            $unit = __('days', 'linuxfoo-gitlab');
+        }
+        elseif($unit=='months') {
+            $unit = __('months', 'linuxfoo-gitlab');
+        }
+        elseif($unit=='years') {
+            $unit = __('years', 'linuxfoo-gitlab');
+        }
+
+        /* translators: %d: count, %s unit of time interval*/
+        return sprintf(__('since %d %s ago', 'linuxfoo-gitlab'), $num, $unit);
+    }
+
 	static function list_commits($atts, $content, $tag) {
 		global $post;
 
@@ -35,6 +58,15 @@ class LinuxfooGitlab {
         ) {
             return self::error(__('Required parameter missing.', 'linuxfoo-gitlab'));
         }
+        elseif(!preg_match('/^[1-9][0-9]*$/', $atts['project_id'])) {
+            return self::error(__('Invalid value for parameter "project_id".', 'linuxfoo-gitlab'));
+        }
+        elseif(!filter_var($atts['url'], FILTER_VALIDATE_URL)) {
+            return self::error(__('Invalid value for parameter "url".', 'linuxfoo-gitlab'));
+        }
+        elseif(!preg_match('/^[1-9][0-9]*$/', $atts['project_id'])) {
+            return self::error(__('Invalid value for parameter "project_id".', 'linuxfoo-gitlab'));
+        }
 
         $project_url = $atts['url'].'/api/v4/projects/'.$atts['project_id'];
         $project_json = file_get_contents($project_url);
@@ -51,12 +83,16 @@ class LinuxfooGitlab {
 
         $project_header =
             '<span class="project_header">'.
-                'Gitlab project'.' <a href="'.$project->web_url.'">'.$project->name.'</a>'.
+                __('Gitlab project', 'linuxfoo-gitlab').' <a href="'.$project->web_url.'">'.$project->name.'</a>'.
             '</span>';
 
         $commits_url = $project_url.'/repository/commits?';
 
         if(!is_null($atts['since'])) {
+            if(!preg_match('/^[1-9][0-9]*\s+(days|months|years)$/', $atts['since'])) {
+                return self::error(__('Invalid value for parameter "since".', 'linuxfoo-gitlab'));
+            }
+
             $since = date(DATE_ISO8601, strtotime('-'.$atts['since']));
             $commits_url .= '&since='.$since;
         }
@@ -95,7 +131,7 @@ class LinuxfooGitlab {
                     '<span class="stats">'.
                         __('No commits', 'linuxfoo-gitlab').
                         (is_null($branch) ? '' : ' '.__('in branch', 'linuxfoo-gitlab').' "'.$branch.'"').
-                        (is_null($atts['since']) ? '' : ' since '.$atts['since'].' ago').'.'.
+                        (is_null($atts['since']) ? '' : ' '.('since '.$atts['since'].' ago')).'.'.
                     '</span>'.
                 '</div>';
         }
@@ -106,9 +142,10 @@ class LinuxfooGitlab {
             '<div class="linuxfoo_gitlab">'.
                 $project_header.
                     '<span class="stats">'.
+                        /* translators: %d: maximum number of commits displayed */
                         sprintf(__('Last %d commits', 'linuxfoo-gitlab'), $commits_count).
                         (is_null($branch) ? '' : ' '.__('in branch', 'linuxfoo-gitlab').' "'.$branch.'"').
-                        (is_null($atts['since']) ? '' : ' since '.$atts['since'].' ago').':'.
+                        (is_null($atts['since']) ? '' : ' '.self::format_since($atts['since'])).':'.
                     '</span>'.
                 '<ul class="commits">';
 
@@ -130,10 +167,6 @@ class LinuxfooGitlab {
 
         return $out;
     }
-
-    static function load_textdomain() {
-        load_plugin_textdomain('linuxfoo-gitlab', false, dirname(plugin_basename(__FILE__)).'/lang');
-    }
 }
 
 add_action('plugins_loaded', 'LinuxfooGitlab::load_textdomain');