diff --git a/composer.json b/composer.json
index 858fa7e..fafaa03 100644
--- a/composer.json
+++ b/composer.json
@@ -32,6 +32,7 @@
         "illuminate/console": "^6.0 || ^7.0 || ^8.0 || ^9.0",
         "illuminate/database": "^6.0 || ^7.0 || ^8.0 || ^9.0",
         "illuminate/support": "^6.0 || ^7.0 || ^8.0 || ^9.0",
+        "lmc/http-constants": "^1.2",
         "nesbot/carbon": "^1.20 || ^2.0",
         "psr/http-message": "^1.0.1"
     },
diff --git a/src/Services/Checker.php b/src/Services/Checker.php
index 6249b5c..dbe18b9 100644
--- a/src/Services/Checker.php
+++ b/src/Services/Checker.php
@@ -24,6 +24,7 @@
 use DragonCode\LastModified\Concerns\Urlable;
 use DragonCode\Support\Concerns\Makeable;
 use Illuminate\Http\Request;
+use Lmc\HttpConstants\Header;
 
 /**
  * @method static Checker make(Request $request = null)
@@ -83,7 +84,7 @@ protected function find(): ?DateTimeInterface
 
     protected function modifiedSince()
     {
-        return $this->request->headers->getDate('If-Modified-Since');
+        return $this->request->headers->getDate(Header::IF_MODIFIED_SINCE);
     }
 
     protected function doesntRequest(): bool
diff --git a/tests/Concerns/Requests.php b/tests/Concerns/Requests.php
index b02acc3..e8f899c 100644
--- a/tests/Concerns/Requests.php
+++ b/tests/Concerns/Requests.php
@@ -21,6 +21,7 @@
 
 use Carbon\Carbon;
 use Illuminate\Http\Request;
+use Lmc\HttpConstants\Header;
 
 trait Requests
 {
@@ -33,7 +34,7 @@ trait Requests
     protected function request(string $url, Carbon $date = null)
     {
         if (! empty($date)) {
-            $this->withHeader('If-Modified-Since', $date->format('r'));
+            $this->withHeader(Header::IF_MODIFIED_SINCE, $date->format('r'));
         }
 
         return $this->get($url);