diff --git a/src/Generators/BaseOutlook.php b/src/Generators/BaseOutlook.php
index 69f5f2f..48de855 100644
--- a/src/Generators/BaseOutlook.php
+++ b/src/Generators/BaseOutlook.php
@@ -12,23 +12,23 @@
 abstract class BaseOutlook implements Generator
 {
     /** @see https://www.php.net/manual/en/function.date.php */
-    protected string $dateFormat = 'Y-m-d';
+    private const DATE_FORMAT = 'Y-m-d';
 
     /** @see https://www.php.net/manual/en/function.date.php */
-    protected string $dateTimeFormat = 'Y-m-d\TH:i:s\Z';
+    private const DATETIME_FORMAT = 'Y-m-d\TH:i:s\Z';
 
     /**
      * Get base URL for links.
      * @return non-empty-string
      */
-    abstract public function baseUrl(): string;
+    abstract protected function baseUrl(): string;
 
     /** @inheritDoc */
     public function generate(Link $link): string
     {
         $url = $this->baseUrl();
 
-        $dateTimeFormat = $link->allDay ? $this->dateFormat : $this->dateTimeFormat;
+        $dateTimeFormat = $link->allDay ? self::DATE_FORMAT : self::DATETIME_FORMAT;
 
         $utcStartDateTime = $link->from->setTimezone(new DateTimeZone('UTC'));
         $utcEndDateTime = $link->to->setTimezone(new DateTimeZone('UTC'));
diff --git a/src/Generators/Google.php b/src/Generators/Google.php
index 7f716da..f79df36 100644
--- a/src/Generators/Google.php
+++ b/src/Generators/Google.php
@@ -12,18 +12,22 @@
 class Google implements Generator
 {
     /** @see https://www.php.net/manual/en/function.date.php */
-    protected string $dateFormat = 'Ymd';
+    private const DATE_FORMAT = 'Ymd';
 
-    protected string $dateTimeFormat = 'Ymd\THis\Z';
+    /** @see https://www.php.net/manual/en/function.date.php */
+    private const DATETIME_FORMAT = 'Ymd\THis\Z';
+
+    /** @var non-empty-string */
+    protected const BASE_URL = 'https://calendar.google.com/calendar/render?action=TEMPLATE';
 
     /** @inheritDoc */
     public function generate(Link $link): string
     {
-        $url = 'https://calendar.google.com/calendar/render?action=TEMPLATE';
+        $url = self::BASE_URL;
 
         $utcStartDateTime = $link->from->setTimezone(new DateTimeZone('UTC'));
         $utcEndDateTime = $link->to->setTimezone(new DateTimeZone('UTC'));
-        $dateTimeFormat = $link->allDay ? $this->dateFormat : $this->dateTimeFormat;
+        $dateTimeFormat = $link->allDay ? self::DATE_FORMAT : self::DATETIME_FORMAT;
         $url .= '&dates='.$utcStartDateTime->format($dateTimeFormat).'/'.$utcEndDateTime->format($dateTimeFormat);
 
         // Add timezone name if it is specified in both from and to dates and is the same for both
diff --git a/src/Generators/WebOffice.php b/src/Generators/WebOffice.php
index 1462368..14f8783 100644
--- a/src/Generators/WebOffice.php
+++ b/src/Generators/WebOffice.php
@@ -2,13 +2,13 @@
 
 namespace Spatie\CalendarLinks\Generators;
 
-class WebOffice extends BaseOutlook
+final class WebOffice extends BaseOutlook
 {
     /** @var non-empty-string */
-    protected const BASE_URL = 'https://outlook.office.com/calendar/deeplink/compose?path=/calendar/action/compose&rru=addevent';
+    private const BASE_URL = 'https://outlook.office.com/calendar/deeplink/compose?path=/calendar/action/compose&rru=addevent';
 
     /** @inheritDoc */
-    public function baseUrl(): string
+    protected function baseUrl(): string
     {
         return static::BASE_URL;
     }
diff --git a/src/Generators/WebOutlook.php b/src/Generators/WebOutlook.php
index 8316dae..c1ebb33 100644
--- a/src/Generators/WebOutlook.php
+++ b/src/Generators/WebOutlook.php
@@ -2,13 +2,13 @@
 
 namespace Spatie\CalendarLinks\Generators;
 
-class WebOutlook extends BaseOutlook
+final class WebOutlook extends BaseOutlook
 {
     /** @var non-empty-string */
-    protected const BASE_URL = 'https://outlook.live.com/calendar/action/compose?path=/calendar/action/compose&rru=addevent';
+    private const BASE_URL = 'https://outlook.live.com/calendar/action/compose?path=/calendar/action/compose&rru=addevent';
 
     /** @inheritDoc */
-    public function baseUrl(): string
+    protected function baseUrl(): string
     {
         return static::BASE_URL;
     }
diff --git a/src/Generators/Yahoo.php b/src/Generators/Yahoo.php
index e11f041..8531c8c 100644
--- a/src/Generators/Yahoo.php
+++ b/src/Generators/Yahoo.php
@@ -12,16 +12,20 @@
 class Yahoo implements Generator
 {
     /** @see https://www.php.net/manual/en/function.date.php */
-    protected string $dateFormat = 'Ymd';
+    private const DATE_FORMAT = 'Ymd';
 
-    protected string $dateTimeFormat = 'Ymd\THis\Z';
+    /** @see https://www.php.net/manual/en/function.date.php */
+    private const DATETIME_FORMAT = 'Ymd\THis\Z';
+
+    /** @var non-empty-string */
+    protected const BASE_URL = 'https://calendar.yahoo.com/?v=60&view=d&type=20';
 
     /** @inheritDoc */
     public function generate(Link $link): string
     {
-        $url = 'https://calendar.yahoo.com/?v=60&view=d&type=20';
+        $url = self::BASE_URL;
 
-        $dateTimeFormat = $link->allDay ? $this->dateFormat : $this->dateTimeFormat;
+        $dateTimeFormat = $link->allDay ? self::DATE_FORMAT : self::DATETIME_FORMAT;
 
         if ($link->allDay) {
             $url .= '&ST='.$link->from->format($dateTimeFormat);