Stringify years correctly according to ISO 8601

src/Year.php

@@ -10,6 +10,10 @@
 use Brick\DateTime\Parser\IsoParsers;
 use JsonSerializable;
 
+use function str_pad;
+
+use const STR_PAD_LEFT;
+
 /**
  * Represents a year in the proleptic calendar.
  */
@@ -281,7 +285,14 @@ public function jsonSerialize(): string
      */
     public function toISOString(): string
     {
-        return (string) $this->year;
+        // This code is optimized for high performance
+        return $this->year < 1000 && $this->year > -1000
+            ? (
+                $this->year < 0
+                ? '-' . str_pad((string) -$this->year, 4, '0', STR_PAD_LEFT)
+                : str_pad((string) $this->year, 4, '0', STR_PAD_LEFT)
+            )
+            : (string) $this->year;
     }
 
     /**

tests/YearTest.php

@@ -516,7 +516,9 @@ public function testToString(int $year, string $expectedString): void
     public function providerToString(): array
     {
         return [
-            [-100, '-100'],
+            [-100, '-0100'],
+            [100, '0100'],
+            [0, '0000'],
             [1987, '1987'],
             [105781, '105781'],
         ];