Fix strange behavior of PHP easter_date function

src/ChristianCalendar.php

@@ -4,8 +4,6 @@
 
 final class ChristianCalendar
 {
-    private const MAX_PHP_EASTER_YEAR = 2037;
-
     public function getEasterFriday(int $year): \DateTimeImmutable
     {
         return $this->getDateAroundEaster($year, -2);
@@ -28,28 +26,23 @@ public function getAscensionDay(int $year): \DateTimeImmutable
 
     private function getDateAroundEaster(int $year, int $nbDaysAfterEaster): \DateTimeImmutable
     {
-        $easter = $this->getEasterDate($year);
-
-        if ($nbDaysAfterEaster === 0) {
-            return $easter;
-        }
-
-        $interval = new \DateInterval('P' . abs($nbDaysAfterEaster) . 'D');
-
-        return $nbDaysAfterEaster > 0 ? $easter->add($interval) : $easter->sub($interval);
+        return $this->addDays($this->getEasterDate($year), $nbDaysAfterEaster);
     }
 
+    /** @see <https://www.php.net/manual/en/function.easter-date.php#refsect1-function.easter-date-notes> */
     private function getEasterDate(int $year): \DateTimeImmutable
     {
-        return \DateTimeImmutable::createFromFormat('U', easter_date($this->guardEasterYear($year)));
+        return $this->addDays(new \DateTimeImmutable($year . '-03-21'), easter_days($year));
     }
 
-    private function guardEasterYear(int $year): int
+    private function addDays(\DateTimeImmutable $date, int $nbDays): \DateTimeImmutable
     {
-        if ($year > self::MAX_PHP_EASTER_YEAR) {
-            throw new \InvalidArgumentException('Easter date not found');
+        if ($nbDays === 0) {
+            return $date;
         }
 
-        return $year;
+        $interval = new \DateInterval('P' . abs($nbDays) . 'D');
+
+        return $nbDays > 0 ? $date->add($interval) : $date->sub($interval);
     }
 }

test/Integration/StaticFRBusinessCalendarTest.php

@@ -7,16 +7,6 @@
 
 class StaticFRBusinessCalendarTest extends FRBusinessCalendarTest
 {
-    /**
-     * @test
-     */
-    public function whenYearIsBeyondPHPThenThrowException()
-    {
-        $this->expectException(\InvalidArgumentException::class);
-
-        $this->add('2036/01/01', 720);
-    }
-
     /**
      * @test
      */

test/Unit/ChristianCalendarTest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Nati\Businesscal\Test\Unit;
+
+use Nati\Businesscal\ChristianCalendar;
+use PHPUnit\Framework\TestCase;
+
+final class ChristianCalendarTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function canReturnEasterDateBasedHolidayWhateverTZ()
+    {
+        //The purpose of this test is to fix strange behavior of easter_date PHP function
+
+        putenv('TZ=UTC');
+        $this->assertEquals('18/05', (new ChristianCalendar())->getAscensionDay(2023)->format('d/m'));
+
+        putenv('TZ=Europe/Paris');
+        $this->assertEquals('18/05', (new ChristianCalendar())->getAscensionDay(2023)->format('d/m'));
+    }
+
+    /**
+     * @test
+     */
+    public function canReturnEasterDateBasedHolidayEvenInDistantFuture()
+    {
+        //The purpose of this test is to fix strange behavior of easter_date PHP function
+
+        $this->assertEquals('18/05', (new ChristianCalendar())->getAscensionDay(2045)->format('d/m'));
+    }
+}