diff --git a/src/System/Time/Now.php b/src/System/Time/Now.php
index 2279bdee..c35739fc 100644
--- a/src/System/Time/Now.php
+++ b/src/System/Time/Now.php
@@ -9,18 +9,18 @@
 use System\Time\Traits\DateTimeFormatTrait;
 
 /**
- * @property int       $timestamp
- * @property int       $year
- * @property int       $month
- * @property int       $day
- * @property int       $hour
- * @property int       $minute
- * @property int       $second
- * @property string    $monthName
- * @property string    $dayName
- * @property string    $shortDay
- * @property string    $timeZone
- * @property int|float $age
+ * @property int    $timestamp
+ * @property int    $year
+ * @property int    $month
+ * @property int    $day
+ * @property int    $hour
+ * @property int    $minute
+ * @property int    $second
+ * @property string $monthName
+ * @property string $dayName
+ * @property string $shortDay
+ * @property string $timeZone
+ * @property int    $age
  */
 class Now
 {
@@ -54,8 +54,7 @@ class Now
     private $timeZone;
 
     // other property
-    /** @var int|float */
-    private $age;
+    private int $age;
 
     public function __construct(string $date_format = 'now', ?string $time_zone = null)
     {
@@ -126,9 +125,7 @@ private function refresh(): void
         $this->timeZone  = $this->date->format('e');
         $this->shortDay  = $this->date->format('D');
 
-        $now       = new \DateTime('now', new \DateTimeZone($this->timeZone));
-        $interval  = $now->diff($this->date);
-        $this->age = $interval->y;
+        $this->age = max(0, (int) floor((time() - $this->timestamp) / (365.25 * 24 * 60 * 60)));
     }
 
     private function current(string $format, int $timestamp): string
diff --git a/tests/Time/TimeTravelTest.php b/tests/Time/TimeTravelTest.php
index 44115c21..7ffb196f 100644
--- a/tests/Time/TimeTravelTest.php
+++ b/tests/Time/TimeTravelTest.php
@@ -150,14 +150,25 @@ public function itSameWithCustumeTime(): void
         $this->assertTrue($now->isSaturday(), 'day must same');
     }
 
-    /**
-     * @test
-     */
-    public function itCorrectAge(): void
+    /** @test */
+    public function itCalculatesAgeCorrectlyForTypicalBirthday(): void
     {
-        $now         = new Now('01/01/2000');
+        $now         = new Now('01/01/1990');
         $currentYear = (int) date('Y');
-        $expectedAge = $currentYear - 2000;
+        $expectedAge = $currentYear - 1990;
+        $this->assertSame(
+            $expectedAge,
+            $now->age,
+            'the age must equal'
+        );
+    }
+
+    /** @test */
+    public function itHandlesLeapYearBirthdayCorrectly(): void
+    {
+        $now             = new Now('02/29/2000');
+        $birthDateBefore = new DateTime(date('02/29/2000'));
+        $expectedAge     = $birthDateBefore->diff(new DateTime())->y;
         $this->assertSame(
             $expectedAge,
             $now->age,
@@ -165,6 +176,71 @@ public function itCorrectAge(): void
         );
     }
 
+    /** @test */
+    public function itHandlesFutureBirthdateCorrectly(): void
+    {
+        $now = new Now('next day');
+        $this->assertSame(
+            0,
+            $now->age,
+            'the age must be 0 for a future birthdate'
+        );
+    }
+
+    /** @test */
+    public function itCalculatesAgeAsZeroForTodayBirthdate(): void
+    {
+        $now = new Now('now', 'utc');
+        $this->assertSame(
+            0,
+            $now->age,
+            'the age must be 0 for today\'s birthdate'
+        );
+    }
+
+    /** @test */
+    public function itHandlesEdgeCasesAroundBirthdayCorrectly(): void
+    {
+        $nowBeforeBirthday = new Now(date('m/d/Y', strtotime('-1 day')));
+        $birthDateBefore   = new DateTime(date('m/d/Y', strtotime('-1 day')));
+        $expectedAgeBefore = $birthDateBefore->diff(new DateTime())->y;
+        $this->assertSame(
+            $expectedAgeBefore,
+            $nowBeforeBirthday->age,
+            'the age must be correct just before the birthday'
+        );
+
+        $nowAfterBirthday = new Now(date('m/d/Y', strtotime('+1 day')));
+        $birthDateAfter   = new DateTime(date('m/d/Y', strtotime('+1 day')));
+        $expectedAgeAfter = $birthDateAfter->diff(new DateTime())->y;
+        $this->assertSame(
+            $expectedAgeAfter,
+            $nowAfterBirthday->age,
+            'the age must be correct just after the birthday'
+        );
+    }
+
+    /** @test */
+    public function itHandlesDifferentTimeZonesCorrectly(): void
+    {
+        $nowUTC      = new Now('01/01/2000', 'UTC');
+        $nowJKT      = new Now('01/01/2000', 'Asia/Jakarta');
+        $birthDate   = new DateTime('01/01/2000');
+        $expectedAge = $birthDate->diff(new DateTime())->y;
+
+        $this->assertSame(
+            $expectedAge,
+            $nowUTC->age,
+            'the age must be correct in UTC'
+        );
+
+        $this->assertSame(
+            $expectedAge,
+            $nowJKT->age,
+            'the age must be correct in Asia/Jakarta'
+        );
+    }
+
     /** @test */
     public function itCanGetFromPrivateProperty()
     {