diff --git a/CHANGELOG.md b/CHANGELOG.md
index f59a4807..8499c1a1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## 2.6.1 under development
 
+- Bug #819: Preserve microsecond precision when comparing date values in `UniqueIterable` (@samdark)
 - Bug #817: Fix `UniqueIterable` comparing the whole iterable instead of its items (@CorvusSharp)
 
 ## 2.6.0 June 02, 2026
diff --git a/src/Rule/UniqueIterableHandler.php b/src/Rule/UniqueIterableHandler.php
index 7f20739c..7f35c885 100644
--- a/src/Rule/UniqueIterableHandler.php
+++ b/src/Rule/UniqueIterableHandler.php
@@ -60,7 +60,8 @@ public function validate(mixed $value, RuleInterface $rule, ValidationContext $c
             if ($item instanceof Stringable) {
                 $stack[] = (string) $item;
             } elseif ($item instanceof DateTimeInterface) {
-                $stack[] = $item->getTimestamp();
+                // Use a non-numeric separator to preserve microseconds during comparison.
+                $stack[] = $item->format('U:u');
             } else {
                 $stack[] = $item;
             }
diff --git a/tests/Rule/UniqueIterableTest.php b/tests/Rule/UniqueIterableTest.php
index ac6031d7..f925e426 100644
--- a/tests/Rule/UniqueIterableTest.php
+++ b/tests/Rule/UniqueIterableTest.php
@@ -5,6 +5,7 @@
 namespace Yiisoft\Validator\Tests\Rule;
 
 use DateTime;
+use DateTimeImmutable;
 use stdClass;
 use Stringable;
 use Yiisoft\Validator\PropertyTranslator\ArrayPropertyTranslator;
@@ -120,6 +121,20 @@ public function __toString()
                 [new DateTime('2024-04-10 14:05:01'), new DateTime('2024-04-10 14:05:02')],
                 new UniqueIterable(),
             ],
+            'datetime values with different microseconds' => [
+                [
+                    new DateTime('2024-04-10 14:05:01.000001 UTC'),
+                    new DateTime('2024-04-10 14:05:01.000002 UTC'),
+                ],
+                new UniqueIterable(),
+            ],
+            'distant datetime values with different microseconds' => [
+                [
+                    new DateTimeImmutable('9999-04-10 14:05:01.000001 UTC'),
+                    new DateTimeImmutable('9999-04-10 14:05:01.000002 UTC'),
+                ],
+                new UniqueIterable(),
+            ],
             'more than two unique strings' => [['a', 'b', 'c'], new UniqueIterable()],
             'more than two unique integers' => [[1, 2, 3, 4], new UniqueIterable()],
             'more than two unique floats' => [[1.5, 2.5, 3.5], new UniqueIterable()],
@@ -284,6 +299,14 @@ public function __toString()
                 new UniqueIterable(),
                 ['' => [$message]],
             ],
+            'equal datetime values with microseconds in different timezones' => [
+                [
+                    new DateTime('2024-04-10 14:05:01.123456 +00:00'),
+                    new DateTime('2024-04-10 17:05:01.123456 +03:00'),
+                ],
+                new UniqueIterable(),
+                ['' => [$message]],
+            ],
             'strings' => [['a', 'b', 'a', 'c'], new UniqueIterable(), ['' => [$message]]],
             'integers' => [[1, 2, 1, 3], new UniqueIterable(), ['' => [$message]]],
             'floats' => [[1.5, 2.5, 1.5, 3.5], new UniqueIterable(), ['' => [$message]]],