diff --git a/src/EvaluationContextConverter.php b/src/EvaluationContextConverter.php
index 135ae89..0300e1f 100644
--- a/src/EvaluationContextConverter.php
+++ b/src/EvaluationContextConverter.php
@@ -5,6 +5,9 @@
 namespace LaunchDarkly\OpenFeature;
 
 use DateTime;
+use DateTimeImmutable;
+use DateTimeInterface;
+use DateTimeZone;
 use LaunchDarkly\LDContext;
 use LaunchDarkly\LDContextBuilder;
 use LaunchDarkly\LDContextMultiBuilder;
@@ -157,10 +160,39 @@ private function buildSingleContext(array $attributes, string $kind, string $key
                 $this->logger->error("'privateAttributes' in an evaluation context must be an array");
             } else {
                 // Catch all for remaining attributes
-                $builder->set($k, $v);
+                $builder->set((string) $k, $this->toLdValue($v, (string) $k));
             }
         }
 
         return $builder->build();
     }
+
+    /**
+     * Convert an OpenFeature attribute value into a value the LaunchDarkly
+     * context model can hold. Dates become ISO-8601 strings in UTC, which
+     * the LaunchDarkly date operators accept.
+     */
+    private function toLdValue(mixed $value, string $attributeName): mixed
+    {
+        if ($value === null || is_bool($value) || is_int($value) || is_float($value) || is_string($value)) {
+            return $value;
+        }
+
+        if ($value instanceof DateTimeInterface) {
+            return DateTimeImmutable::createFromInterface($value)
+                ->setTimezone(new DateTimeZone('UTC'))
+                ->format('Y-m-d\\TH:i:s.v\\Z');
+        }
+
+        if (is_array($value)) {
+            $converted = [];
+            foreach ($value as $k => $v) {
+                $converted[$k] = $this->toLdValue($v, $attributeName);
+            }
+            return $converted;
+        }
+
+        $this->logger->error("The attribute '$attributeName' contained a value which could not be converted; using null");
+        return null;
+    }
 }
diff --git a/tests/EvaluationContextConverterTest.php b/tests/EvaluationContextConverterTest.php
index ad5f157..b70c666 100644
--- a/tests/EvaluationContextConverterTest.php
+++ b/tests/EvaluationContextConverterTest.php
@@ -4,10 +4,14 @@
 
 namespace LaunchDarkly\Tests;
 
+use DateTime;
+use DateTimeImmutable;
+use DateTimeZone;
 use LaunchDarkly\OpenFeature\EvaluationContextConverter;
 use OpenFeature\implementation\flags\Attributes;
 use OpenFeature\implementation\flags\EvaluationContext;
 use PHPUnit\Framework\TestCase;
+use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
 
 class EvaluationContextConverterTest extends TestCase
@@ -208,4 +212,50 @@ public function testHandlesInvalidNestedContexts(): void
         $this->assertFalse($ldContext->isValid());
         $this->assertEquals(0, $ldContext->getIndividualContextCount());
     }
+
+    public function testDateAttributesAreConvertedToUtcIso8601Strings(): void
+    {
+        $attributes = new Attributes([
+            'signupDate' => new DateTime('2024-03-01T09:30:00.123+09:00'),
+            'address' => ['movedIn' => new DateTimeImmutable('2024-03-01T00:00:00', new DateTimeZone('UTC'))],
+            'history' => [new DateTimeImmutable('2023-12-31T23:00:00-01:00'), 'other'],
+        ]);
+        $context = new EvaluationContext('user-key', $attributes);
+
+        $ldContext = $this->contextConverter->toLdContext($context);
+
+        $this->assertTrue($ldContext->isValid());
+        $this->assertSame('2024-03-01T00:30:00.123Z', $ldContext->get('signupDate'));
+        $this->assertSame(['movedIn' => '2024-03-01T00:00:00.000Z'], $ldContext->get('address'));
+        $this->assertSame(['2024-01-01T00:00:00.000Z', 'other'], $ldContext->get('history'));
+    }
+
+    public function testMultiContextDateAttributesAreConverted(): void
+    {
+        $attributes = new Attributes([
+            'kind' => 'multi',
+            'org' => ['key' => 'org-key', 'founded' => new DateTimeImmutable('2001-02-03T04:05:06Z')],
+        ]);
+        $context = new EvaluationContext(null, $attributes);
+
+        $ldContext = $this->contextConverter->toLdContext($context);
+
+        /** @var \LaunchDarkly\LDContext */
+        $orgContext = $ldContext->getIndividualContext('org');
+        $this->assertSame('2001-02-03T04:05:06.000Z', $orgContext->get('founded'));
+    }
+
+    public function testUnconvertibleValuesBecomeNullAndLogAnError(): void
+    {
+        $logger = $this->createMock(LoggerInterface::class);
+        $logger->expects($this->once())->method('error');
+        $converter = new EvaluationContextConverter($logger);
+
+        $attributes = new Attributes(['list' => ['valid', new \stdClass()]]);
+        $context = new EvaluationContext('user-key', $attributes);
+
+        $ldContext = $converter->toLdContext($context);
+
+        $this->assertSame(['valid', null], $ldContext->get('list'));
+    }
 }