diff --git a/src/Field/SecondOfDay.php b/src/Field/SecondOfDay.php
index 40b3a3f..55d10ac 100644
--- a/src/Field/SecondOfDay.php
+++ b/src/Field/SecondOfDay.php
@@ -22,6 +22,8 @@ final class SecondOfDay
      * @return void
      *
      * @throws DateTimeException If the second-of-day is not valid.
+     *
+     * @psalm-pure
      */
     public static function check(int $secondOfDay) : void
     {
diff --git a/src/LocalTime.php b/src/LocalTime.php
index 94ad4b7..29c2b68 100644
--- a/src/LocalTime.php
+++ b/src/LocalTime.php
@@ -109,6 +109,8 @@ public static function of(int $hour, int $minute, int $second = 0, int $nano = 0
      * @return LocalTime
      *
      * @throws DateTimeException
+     *
+     * @psalm-pure
      */
     public static function ofSecondOfDay(int $secondOfDay, int $nanoOfSecond = 0) : LocalTime
     {
diff --git a/src/TimeZoneOffset.php b/src/TimeZoneOffset.php
index 335a5b1..11621fb 100644
--- a/src/TimeZoneOffset.php
+++ b/src/TimeZoneOffset.php
@@ -24,9 +24,7 @@ final class TimeZoneOffset extends TimeZone
     /**
      * The string representation of this time-zone offset.
      *
-     * This is generated on-the-fly, and will be null before the first call to getId().
-     *
-     * @var string|null
+     * @var string
      */
     private $id;
 
@@ -38,6 +36,14 @@ final class TimeZoneOffset extends TimeZone
     private function __construct(int $totalSeconds)
     {
         $this->totalSeconds = $totalSeconds;
+
+        if ($this->totalSeconds < 0) {
+            $this->id = '-' . LocalTime::ofSecondOfDay(- $this->totalSeconds);
+        } elseif ($this->totalSeconds > 0) {
+            $this->id = '+' . LocalTime::ofSecondOfDay($this->totalSeconds);
+        } else {
+            $this->id = 'Z';
+        }
     }
 
     /**
@@ -186,16 +192,6 @@ public function getTotalSeconds() : int
      */
     public function getId() : string
     {
-        if ($this->id === null) {
-            if ($this->totalSeconds < 0) {
-                $this->id = '-' . LocalTime::ofSecondOfDay(- $this->totalSeconds);
-            } elseif ($this->totalSeconds > 0) {
-                $this->id = '+' . LocalTime::ofSecondOfDay($this->totalSeconds);
-            } else {
-                $this->id = 'Z';
-            }
-        }
-
         return $this->id;
     }