diff --git a/src/MonthDay.php b/src/MonthDay.php
index 7b1ddb4..207dae8 100644
--- a/src/MonthDay.php
+++ b/src/MonthDay.php
@@ -10,8 +10,6 @@
 use Brick\DateTime\Parser\IsoParsers;
 use JsonSerializable;
 
-use function sprintf;
-
 /**
  * A month-day in the ISO-8601 calendar system, such as `--12-03`.
  */
@@ -235,6 +233,9 @@ public function jsonSerialize(): string
 
     public function __toString(): string
     {
-        return sprintf('--%02d-%02d', $this->month, $this->day);
+        return '--'
+            . ($this->month < 10 ? '0' . $this->month : $this->month)
+            . '-'
+            . ($this->day < 10 ? '0' . $this->day : $this->day);
     }
 }
diff --git a/src/YearMonth.php b/src/YearMonth.php
index 6c4e1b6..b710e0c 100644
--- a/src/YearMonth.php
+++ b/src/YearMonth.php
@@ -11,7 +11,10 @@
 use Brick\DateTime\Utility\Math;
 use JsonSerializable;
 
-use function sprintf;
+use function abs;
+use function str_pad;
+
+use const STR_PAD_LEFT;
 
 /**
  * Represents the combination of a year and a month.
@@ -300,6 +303,8 @@ public function jsonSerialize(): string
      */
     public function __toString(): string
     {
-        return sprintf('%02u-%02u', $this->year, $this->month);
+        return ($this->year < 1000 ? ($this->year < 0 ? '-' : '') . str_pad((string) abs($this->year), 4, '0', STR_PAD_LEFT) : $this->year)
+            . '-'
+            . ($this->month < 10 ? '0' . $this->month : $this->month);
     }
 }
diff --git a/src/YearWeek.php b/src/YearWeek.php
index 8a410cc..81b55c9 100644
--- a/src/YearWeek.php
+++ b/src/YearWeek.php
@@ -6,7 +6,10 @@
 
 use JsonSerializable;
 
-use function sprintf;
+use function abs;
+use function str_pad;
+
+use const STR_PAD_LEFT;
 
 /**
  * Represents the combination of a year and a week.
@@ -271,8 +274,8 @@ public function jsonSerialize(): string
 
     public function __toString(): string
     {
-        $pattern = ($this->year < 0 ? '%05d' : '%04d') . '-W%02d';
-
-        return sprintf($pattern, $this->year, $this->week);
+        return ($this->year < 1000 ? ($this->year < 0 ? '-' : '') . str_pad((string) abs($this->year), 4, '0', STR_PAD_LEFT) : $this->year)
+            . '-W'
+            . ($this->week < 10 ? '0' . $this->week : $this->week);
     }
 }
diff --git a/tests/YearMonthTest.php b/tests/YearMonthTest.php
index d758741..b28656c 100644
--- a/tests/YearMonthTest.php
+++ b/tests/YearMonthTest.php
@@ -459,13 +459,46 @@ public function providerToLocalDateRange(): array
         ];
     }
 
-    public function testJsonSerialize(): void
+    /**
+     * @dataProvider providerToString
+     */
+    public function testJsonSerialize(int $year, int $week, string $expected): void
+    {
+        $yearWeek = YearMonth::of($year, $week);
+        self::assertSame(json_encode($expected), json_encode($yearWeek));
+    }
+
+    /**
+     * @dataProvider providerToString
+     *
+     * @param int    $year     The year.
+     * @param int    $month    The month.
+     * @param string $expected The expected result string.
+     */
+    public function testToString(int $year, int $month, string $expected): void
     {
-        self::assertSame(json_encode('2013-09'), json_encode(YearMonth::of(2013, 9)));
+        self::assertSame($expected, (string) YearMonth::of($year, $month));
     }
 
-    public function testToString(): void
+    public function providerToString(): array
     {
-        self::assertSame('2013-09', (string) YearMonth::of(2013, 9));
+        return [
+            [-999999, 12, '-999999-12'],
+            [-185321, 11, '-185321-11'],
+            [-18532, 11, '-18532-11'],
+            [-2023, 11, '-2023-11'],
+            [-2023, 11, '-2023-11'],
+            [-2023, 1, '-2023-01'],
+            [-999, 1, '-0999-01'],
+            [-2, 1, '-0002-01'],
+            [2, 1, '0002-01'],
+            [999, 1, '0999-01'],
+            [2023, 1, '2023-01'],
+            [2023, 11, '2023-11'],
+            [2023, 11, '2023-11'],
+            [18532, 11, '18532-11'],
+            [185321, 11, '185321-11'],
+            [999999, 12, '999999-12'],
+        ];
     }
 }