Some random thoughts

src/Time.php

@@ -32,6 +32,8 @@ abstract class Time
         'h'  => self::Hour,
     ];
 
+    private static Time\Duration $_zeroDuration;
+
     private function __construct()
     {
         // not designed to be constructed
@@ -254,6 +256,17 @@ public static function Duration(null|string|int|float|Time\Duration|\DateInterva
         }
     }
 
+    /**
+     * Returns a zero duration instance.
+     */
+    public static function ZeroDuration(): Time\Duration
+    {
+        if (!isset(self::$_zeroDuration)) {
+            self::$_zeroDuration = new Time\Duration(0);
+        }
+        return self::$_zeroDuration;
+    }
+
     private static function _invalidDurationException(string $orig): \InvalidArgumentException
     {
         return new \InvalidArgumentException("Invalid duration: {$orig}");

src/Time/Duration.php

@@ -147,7 +147,7 @@ private static function _fmtFrac(string &$buff, int $v, int $prec): int
         return $v;
     }
 
-    private static function _fmtInt(string &$buff, int $v)
+    private static function _fmtInt(string &$buff, int $v): void
     {
         if (0 === $v) {
             $buff = "0{$buff}";
@@ -159,7 +159,7 @@ private static function _fmtInt(string &$buff, int $v)
         }
     }
 
-    public function jsonSerialize(): mixed
+    public function jsonSerialize(): int
     {
         return $this->ns;
     }

src/Time/Time.php

@@ -13,6 +13,7 @@ class Time extends \DateTime
 
     /**
      * @throws \DateMalformedStringException
+     * @throws \Exception
      */
     #[\ReturnTypeWillChange]
     public static function createFromFormat(string $format, string $datetime, \DateTimeZone $timezone = null): bool|static