diff --git a/src/Duration.php b/src/Duration.php
index 691a54a..fb01785 100644
--- a/src/Duration.php
+++ b/src/Duration.php
@@ -1,5 +1,6 @@
 <?php
-declare(strict_types=1);
+
+declare(strict_types = 1);
 
 namespace MatthewDavis\Duration;
 
diff --git a/src/DurationInterface.php b/src/DurationInterface.php
index 0f0e20f..7b49404 100644
--- a/src/DurationInterface.php
+++ b/src/DurationInterface.php
@@ -1,42 +1,34 @@
 <?php
-declare(strict_types=1);
+
+declare(strict_types = 1);
 
 namespace MatthewDavis\Duration;
 
 interface DurationInterface
 {
     const SECONDS_IN_MINUTE = 60;
-    const SECONDS_IN_HOUR   = 60 * self::SECONDS_IN_MINUTE;
-    const SECONDS_IN_DAY    = 24 * self::SECONDS_IN_HOUR;
-    const SECONDS_IN_WEEK   = 7 * self::SECONDS_IN_DAY;
-
-    public static function second(): DurationInterface;
-
-    public static function seconds(int $seconds): DurationInterface;
-
-    public static function minute(): DurationInterface;
-
-    public static function minutes(int $minutes): DurationInterface;
+    const SECONDS_IN_HOUR = 60 * self::SECONDS_IN_MINUTE;
+    const SECONDS_IN_DAY = 24 * self::SECONDS_IN_HOUR;
+    const SECONDS_IN_WEEK = 7 * self::SECONDS_IN_DAY;
 
-    public static function hour(): DurationInterface;
+    public static function second(): self;
+    public static function seconds(int $seconds): self;
 
-    public static function hours(int $hours): DurationInterface;
+    public static function minute(): self;
+    public static function minutes(int $minutes): self;
 
-    public static function day(): DurationInterface;
+    public static function hour(): self;
+    public static function hours(int $hours): self;
 
-    public static function days(int $days): DurationInterface;
+    public static function day(): self;
+    public static function days(int $days): self;
 
-    public static function week(): DurationInterface;
-
-    public static function weeks(int $weeks): DurationInterface;
+    public static function week(): self;
+    public static function weeks(int $weeks): self;
 
     public function inSeconds(): int;
-
     public function inMinutes(): float;
-
     public function inHours(): float;
-
     public function inDays(): float;
-
     public function inWeeks(): float;
 }
diff --git a/tests/DurationDaysTest.php b/tests/DurationDaysTest.php
index 1d1ec58..104c1c9 100644
--- a/tests/DurationDaysTest.php
+++ b/tests/DurationDaysTest.php
@@ -2,8 +2,8 @@
 
 namespace MatthewDavis\Duration\Test;
 
-use MatthewDavis\Duration\Duration;
 use PHPUnit\Framework\TestCase;
+use MatthewDavis\Duration\Duration;
 
 class DurationDaysTest extends TestCase
 {
@@ -36,7 +36,7 @@ public function it_returns_a_duration_created_in_days_in_hours()
     /** @test */
     public function it_returns_a_duration_created_in_days_in_days()
     {
-        $oneDay    = Duration::days(1)->inDays();
+        $oneDay = Duration::days(1)->inDays();
         $threeDays = Duration::days(3)->inDays();
 
         $this->assertEquals(1, $oneDay);
@@ -46,7 +46,7 @@ public function it_returns_a_duration_created_in_days_in_days()
     /** @test */
     public function it_returns_a_duration_created_in_days_in_weeks()
     {
-        $oneWeek    = Duration::days(7)->inWeeks();
+        $oneWeek = Duration::days(7)->inWeeks();
         $threeWeeks = Duration::days(21)->inWeeks();
 
         $this->assertEquals(1, $oneWeek);
diff --git a/tests/DurationHoursTest.php b/tests/DurationHoursTest.php
index 7ba5534..cb2359b 100644
--- a/tests/DurationHoursTest.php
+++ b/tests/DurationHoursTest.php
@@ -2,8 +2,8 @@
 
 namespace MatthewDavis\Duration\Test;
 
-use MatthewDavis\Duration\Duration;
 use PHPUnit\Framework\TestCase;
+use MatthewDavis\Duration\Duration;
 
 class DurationHoursTest extends TestCase
 {
@@ -26,7 +26,7 @@ public function it_returns_a_duration_created_in_hours_in_minutes()
     /** @test */
     public function it_returns_a_duration_created_in_hours_in_hours()
     {
-        $oneHour    = Duration::hours(1)->inHours();
+        $oneHour = Duration::hours(1)->inHours();
         $threeHours = Duration::hours(3)->inHours();
 
         $this->assertEquals(1, $oneHour);
@@ -36,7 +36,7 @@ public function it_returns_a_duration_created_in_hours_in_hours()
     /** @test */
     public function it_returns_a_duration_created_in_hours_in_days()
     {
-        $oneDay    = Duration::hours(24)->inDays();
+        $oneDay = Duration::hours(24)->inDays();
         $threeDays = Duration::hours(72)->inDays();
 
         $this->assertEquals(1, $oneDay);
@@ -46,7 +46,7 @@ public function it_returns_a_duration_created_in_hours_in_days()
     /** @test */
     public function it_returns_a_duration_created_in_hours_in_weeks()
     {
-        $oneWeek    = Duration::hours(168)->inWeeks();
+        $oneWeek = Duration::hours(168)->inWeeks();
         $threeWeeks = Duration::hours(504)->inWeeks();
 
         $this->assertEquals(1, $oneWeek);
diff --git a/tests/DurationMinutesTest.php b/tests/DurationMinutesTest.php
index c114b75..5cff903 100644
--- a/tests/DurationMinutesTest.php
+++ b/tests/DurationMinutesTest.php
@@ -2,8 +2,8 @@
 
 namespace MatthewDavis\Duration\Test;
 
-use MatthewDavis\Duration\Duration;
 use PHPUnit\Framework\TestCase;
+use MatthewDavis\Duration\Duration;
 
 class DurationMinutesTest extends TestCase
 {
@@ -26,7 +26,7 @@ public function it_returns_a_duration_created_in_minutes_in_minutes()
     /** @test */
     public function it_returns_a_duration_created_in_minutes_in_hours()
     {
-        $oneHour    = Duration::minutes(60)->inHours();
+        $oneHour = Duration::minutes(60)->inHours();
         $threeHours = Duration::minutes(180)->inHours();
 
         $this->assertEquals(1, $oneHour);
@@ -36,7 +36,7 @@ public function it_returns_a_duration_created_in_minutes_in_hours()
     /** @test */
     public function it_returns_a_duration_created_in_minutes_in_days()
     {
-        $oneDay    = Duration::minutes(1440)->inDays();
+        $oneDay = Duration::minutes(1440)->inDays();
         $threeDays = Duration::minutes(4320)->inDays();
 
         $this->assertEquals(1, $oneDay);
@@ -46,7 +46,7 @@ public function it_returns_a_duration_created_in_minutes_in_days()
     /** @test */
     public function it_returns_a_duration_created_in_minutes_in_weeks()
     {
-        $oneWeek    = Duration::minutes(10080)->inWeeks();
+        $oneWeek = Duration::minutes(10080)->inWeeks();
         $threeWeeks = Duration::minutes(30240)->inWeeks();
 
         $this->assertEquals(1, $oneWeek);
diff --git a/tests/DurationSecondsTest.php b/tests/DurationSecondsTest.php
index 7eb19e3..7489170 100644
--- a/tests/DurationSecondsTest.php
+++ b/tests/DurationSecondsTest.php
@@ -2,8 +2,8 @@
 
 namespace MatthewDavis\Duration\Test;
 
-use MatthewDavis\Duration\Duration;
 use PHPUnit\Framework\TestCase;
+use MatthewDavis\Duration\Duration;
 
 class DurationSecondsTest extends TestCase
 {
@@ -18,7 +18,7 @@ public function it_returns_a_duration_created_in_seconds_in_seconds()
     /** @test */
     public function it_returns_a_duration_created_in_seconds_in_minutes()
     {
-        $oneMinute   = Duration::seconds(60)->inMinutes();
+        $oneMinute = Duration::seconds(60)->inMinutes();
         $fiveMinutes = Duration::seconds(300)->inMinutes();
 
         $this->assertEquals(1, $oneMinute);
@@ -28,7 +28,7 @@ public function it_returns_a_duration_created_in_seconds_in_minutes()
     /** @test */
     public function it_returns_a_duration_created_in_seconds_in_hours()
     {
-        $oneHour    = Duration::seconds(3600)->inHours();
+        $oneHour = Duration::seconds(3600)->inHours();
         $threeHours = Duration::seconds(10800)->inHours();
 
         $this->assertEquals(1, $oneHour);
@@ -38,7 +38,7 @@ public function it_returns_a_duration_created_in_seconds_in_hours()
     /** @test */
     public function it_returns_a_duration_created_in_seconds_in_days()
     {
-        $oneDay    = Duration::seconds(86400)->inDays();
+        $oneDay = Duration::seconds(86400)->inDays();
         $threeDays = Duration::seconds(259200)->inDays();
 
         $this->assertEquals(1, $oneDay);
@@ -48,7 +48,7 @@ public function it_returns_a_duration_created_in_seconds_in_days()
     /** @test */
     public function it_returns_a_duration_created_in_seconds_in_weeks()
     {
-        $oneWeek    = Duration::seconds(604800)->inWeeks();
+        $oneWeek = Duration::seconds(604800)->inWeeks();
         $threeWeeks = Duration::seconds(1814400)->inWeeks();
 
         $this->assertEquals(1, $oneWeek);
diff --git a/tests/DurationWeeksTest.php b/tests/DurationWeeksTest.php
index 5925f0d..07871b0 100644
--- a/tests/DurationWeeksTest.php
+++ b/tests/DurationWeeksTest.php
@@ -2,8 +2,8 @@
 
 namespace MatthewDavis\Duration\Test;
 
-use MatthewDavis\Duration\Duration;
 use PHPUnit\Framework\TestCase;
+use MatthewDavis\Duration\Duration;
 
 class DurationWeeksTest extends TestCase
 {
@@ -26,7 +26,7 @@ public function it_returns_a_duration_created_in_weeks_in_minutes()
     /** @test */
     public function it_returns_a_duration_created_in_weeks_in_hours()
     {
-        $oneWeekInHours    = Duration::weeks(1)->inHours();
+        $oneWeekInHours = Duration::weeks(1)->inHours();
         $threeWeeksInHours = Duration::weeks(3)->inHours();
 
         $this->assertEquals(168, $oneWeekInHours);
@@ -36,7 +36,7 @@ public function it_returns_a_duration_created_in_weeks_in_hours()
     /** @test */
     public function it_returns_a_duration_created_in_weeks_in_days()
     {
-        $oneWeekInDays    = Duration::weeks(1)->inDays();
+        $oneWeekInDays = Duration::weeks(1)->inDays();
         $threeWeeksInDays = Duration::weeks(3)->inDays();
 
         $this->assertEquals(7, $oneWeekInDays);
@@ -46,7 +46,7 @@ public function it_returns_a_duration_created_in_weeks_in_days()
     /** @test */
     public function it_returns_a_duration_created_in_weeks_in_weeks()
     {
-        $oneWeek    = Duration::weeks(1)->inWeeks();
+        $oneWeek = Duration::weeks(1)->inWeeks();
         $threeWeeks = Duration::weeks(3)->inWeeks();
 
         $this->assertEquals(1, $oneWeek);