diff --git a/src/CarbonFiscalYearTrait.php b/src/CarbonFiscalYearTrait.php
index 069c0ca..f0a3fa8 100644
--- a/src/CarbonFiscalYearTrait.php
+++ b/src/CarbonFiscalYearTrait.php
@@ -2,6 +2,8 @@
 
 namespace LaracraftTech\CarbonExtensions;
 
+use Carbon\CarbonInterface;
+
 trait CarbonFiscalYearTrait
 {
     /**
@@ -36,7 +38,7 @@ private function ensureFiscalYearStartIsSet()
         }
     }
 
-    public function startOfYear(): CarbonFiscalYear
+    public function startOfYear(): CarbonInterface
     {
         $year = $this->year;
 
@@ -49,7 +51,7 @@ public function startOfYear(): CarbonFiscalYear
         return $this->setDate($year, self::$fiscalYearStartMonth, self::$fiscalYearStartDay)->startOfDay();
     }
 
-    public function endOfYear(): CarbonFiscalYear
+    public function endOfYear(): CarbonInterface
     {
         return $this->startOfYear()->copy()->addYear()->subDay()->endOfDay();
     }
diff --git a/tests/CarbonFiscalYearTest.php b/tests/CarbonFiscalYearTest.php
index 8e6a5fd..9d7e799 100644
--- a/tests/CarbonFiscalYearTest.php
+++ b/tests/CarbonFiscalYearTest.php
@@ -4,23 +4,29 @@
 use LaracraftTech\CarbonExtensions\CarbonFiscalYearImmutable;
 
 it('can detect fiscal year start and end', function () {
-    //set fiscal year
-    CarbonFiscalYear::setFiscalYearStart(4, 1);
+    // Let's test both instances...
+    foreach ([CarbonFiscalYear::class, CarbonFiscalYearImmutable::class] as $dateClass) {
+        /** @var CarbonFiscalYear|CarbonFiscalYearImmutable $dateClass */
+
+        //set fiscal year
+        $dateClass::setFiscalYearStart(4, 1);
+
 
-    $date = CarbonFiscalYear::parse("2022-03-30");
-    expect($date->startOfYear()->format("Y-m-d"))->toBe("2021-04-01")
-        ->and($date->endOfYear()->format("Y-m-d"))->toBe("2022-03-31");
+        $date = $dateClass::parse("2022-03-30");
+        expect($date->startOfYear()->format("Y-m-d"))->toBe("2021-04-01")
+            ->and($date->endOfYear()->format("Y-m-d"))->toBe("2022-03-31");
 
-    $date = CarbonFiscalYear::parse("2022-04-02");
-    expect($date->startOfYear()->format("Y-m-d"))->toBe("2022-04-01")
-        ->and($date->endOfYear()->format("Y-m-d"))->toBe("2023-03-31");
+        $date = $dateClass::parse("2022-04-02");
+        expect($date->startOfYear()->format("Y-m-d"))->toBe("2022-04-01")
+            ->and($date->endOfYear()->format("Y-m-d"))->toBe("2023-03-31");
 
-    //another fiscal year
-    CarbonFiscalYear::setFiscalYearStart(10, 1);
+        //another fiscal year
+        $dateClass::setFiscalYearStart(10, 1);
 
-    $date = CarbonFiscalYear::parse("2023-05-11");
-    expect($date->startOfYear()->format("Y-m-d"))->toBe("2022-10-01")
-        ->and($date->endOfYear()->format("Y-m-d"))->toBe("2023-09-30");
+        $date = $dateClass::parse("2023-05-11");
+        expect($date->startOfYear()->format("Y-m-d"))->toBe("2022-10-01")
+            ->and($date->endOfYear()->format("Y-m-d"))->toBe("2023-09-30");
+    }
 });
 
 it('works with im/mutable', function () {