diff --git a/src/SDK/Trace/AutoRootSpan.php b/src/SDK/Trace/AutoRootSpan.php
index 69790cd61..f675f85bf 100644
--- a/src/SDK/Trace/AutoRootSpan.php
+++ b/src/SDK/Trace/AutoRootSpan.php
@@ -43,12 +43,12 @@ public static function create(ServerRequestInterface $request): void
             Version::VERSION_1_25_0->url(),
         );
         $parent = Globals::propagator()->extract($request->getHeaders());
-        $startTime = array_key_exists('REQUEST_TIME_FLOAT', $request->getServerParams())
-            ? $request->getServerParams()['REQUEST_TIME_FLOAT']
-            : (int) microtime(true);
+        $startTimeInNanos = array_key_exists('REQUEST_TIME_FLOAT', $request->getServerParams())
+            ? (int) ((float) $request->getServerParams()['REQUEST_TIME_FLOAT'] * (float) ClockInterface::NANOS_PER_SECOND)
+            : (int) (microtime(true) * (float) ClockInterface::NANOS_PER_SECOND);
         $span = $tracer->spanBuilder($request->getMethod())
             ->setSpanKind(SpanKind::KIND_SERVER)
-            ->setStartTimestamp((int) ($startTime*ClockInterface::NANOS_PER_SECOND))
+            ->setStartTimestamp($startTimeInNanos)
             ->setParent($parent)
             ->setAttribute(TraceAttributes::URL_FULL, (string) $request->getUri())
             ->setAttribute(TraceAttributes::HTTP_REQUEST_METHOD, $request->getMethod())
diff --git a/tests/Unit/SDK/Trace/AutoRootSpanTest.php b/tests/Unit/SDK/Trace/AutoRootSpanTest.php
index a53859f52..0cfe528ea 100644
--- a/tests/Unit/SDK/Trace/AutoRootSpanTest.php
+++ b/tests/Unit/SDK/Trace/AutoRootSpanTest.php
@@ -5,6 +5,7 @@
 namespace OpenTelemetry\Tests\SDK\Trace;
 
 use Nyholm\Psr7\ServerRequest;
+use OpenTelemetry\API\Common\Time\ClockInterface;
 use OpenTelemetry\API\Instrumentation\Configurator;
 use OpenTelemetry\API\Trace\Propagation\TraceContextPropagator;
 use OpenTelemetry\API\Trace\Span;
@@ -133,6 +134,73 @@ public function test_create(): void
         $scope->detach();
     }
 
+    public function test_create_converts_request_time_float_to_nanoseconds(): void
+    {
+        $requestTimeFloat = 1700000000.5;
+        $expectedNanos = (int) ($requestTimeFloat * (float) ClockInterface::NANOS_PER_SECOND);
+
+        $request = new ServerRequest(
+            'GET',
+            'https://example.com/',
+            [],
+            null,
+            '1.1',
+            ['REQUEST_TIME_FLOAT' => $requestTimeFloat],
+        );
+
+        $span = $this->createMock(SpanInterface::class);
+        $spanBuilder = $this->createMock(SpanBuilderInterface::class);
+        $spanBuilder->method('setSpanKind')->willReturnSelf();
+        $spanBuilder->method('setParent')->willReturnSelf();
+        $spanBuilder->method('setAttribute')->willReturnSelf();
+        $spanBuilder->method('startSpan')->willReturn($span);
+        $spanBuilder
+            ->expects($this->once())
+            ->method('setStartTimestamp')
+            ->with($this->identicalTo($expectedNanos))
+            ->willReturnSelf();
+
+        $this->tracer->method('spanBuilder')->willReturn($spanBuilder);
+
+        AutoRootSpan::create($request);
+
+        Context::storage()->scope()?->detach();
+    }
+
+    public function test_create_start_timestamp_preserves_sub_second_precision_without_request_time_float(): void
+    {
+        $request = new ServerRequest('GET', 'https://example.com/');
+
+        $capturedTimestamp = null;
+        $span = $this->createMock(SpanInterface::class);
+        $spanBuilder = $this->createMock(SpanBuilderInterface::class);
+        $spanBuilder->method('setSpanKind')->willReturnSelf();
+        $spanBuilder->method('setParent')->willReturnSelf();
+        $spanBuilder->method('setAttribute')->willReturnSelf();
+        $spanBuilder->method('startSpan')->willReturn($span);
+        $spanBuilder
+            ->expects($this->once())
+            ->method('setStartTimestamp')
+            ->with($this->callback(function (int $timestamp) use (&$capturedTimestamp): bool {
+                $capturedTimestamp = $timestamp;
+
+                return true;
+            }))
+            ->willReturnSelf();
+
+        $this->tracer->method('spanBuilder')->willReturn($spanBuilder);
+
+        $before = (int) (microtime(true) * (float) ClockInterface::NANOS_PER_SECOND);
+        AutoRootSpan::create($request);
+        $after = (int) (microtime(true) * (float) ClockInterface::NANOS_PER_SECOND);
+
+        $this->assertNotNull($capturedTimestamp);
+        $this->assertGreaterThanOrEqual($before, $capturedTimestamp);
+        $this->assertLessThanOrEqual($after, $capturedTimestamp);
+
+        Context::storage()->scope()?->detach();
+    }
+
     public function test_shutdown_handler(): void
     {
         $this->setEnvironmentVariable('OTEL_PHP_DEBUG_SCOPES_DISABLED', 'true');