Fix iFlytek scoring authentication dates

backend/unispeaking-server/src/main/java/com/unispeaking/infrastructure/ai/iflytek/IflytekScoringProvider.java

@@ -11,12 +11,13 @@
 import java.nio.charset.StandardCharsets;
 import java.security.GeneralSecurityException;
 import java.time.Duration;
+import java.time.Instant;
 import java.time.ZoneOffset;
-import java.time.ZonedDateTime;
 import java.time.format.DateTimeFormatter;
 import java.util.Arrays;
 import java.util.Base64;
 import java.util.LinkedHashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
@@ -52,6 +53,11 @@ public class IflytekScoringProvider extends ScoringProvider {
 			"cn-east-1.ws-api.xf-yun.com";
 	private static final String SUNTONE_PATH =
 			"/v1/private/s8e098720";
+	private static final DateTimeFormatter HTTP_DATE_FORMATTER =
+			DateTimeFormatter.ofPattern(
+					"EEE, dd MMM yyyy HH:mm:ss 'GMT'",
+					Locale.US)
+					.withZone(ZoneOffset.UTC);
 
 	private final ObjectMapper objectMapper;
 	private final WebSocketConnector connector;
@@ -444,8 +450,7 @@ private long remainingNanos(long deadlineNanos)
 	private URI signedEndpoint(String signingApiKey) {
 		try {
 			String host = endpoint.getHost();
-			String date = ZonedDateTime.now(ZoneOffset.UTC)
-					.format(DateTimeFormatter.RFC_1123_DATE_TIME);
+			String date = formatHttpDate(Instant.now());
 			String requestLine =
 					"GET " + endpoint.getRawPath() + " HTTP/1.1";
 			String signatureOrigin =
@@ -482,6 +487,12 @@ private URI signedEndpoint(String signingApiKey) {
 		}
 	}
 
+	static String formatHttpDate(Instant instant) {
+		return HTTP_DATE_FORMATTER.format(Objects.requireNonNull(
+				instant,
+				"iFlytek signature instant is required"));
+	}
+
 	private byte[] wavPayload(byte[] wav) {
 		if (wav.length < 12
 				|| wav[0] != 'R'

backend/unispeaking-server/src/test/java/com/unispeaking/infrastructure/ai/iflytek/IflytekScoringProviderDateTest.java

@@ -0,0 +1,25 @@
+package com.unispeaking.infrastructure.ai.iflytek;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.time.Instant;
+import org.junit.jupiter.api.Test;
+
+class IflytekScoringProviderDateTest {
+
+	@Test
+	void formatsSingleDigitDayWithTheTwoDigitsRequiredByIflytek() {
+		assertEquals(
+				"Sat, 01 Aug 2026 04:54:00 GMT",
+				IflytekScoringProvider.formatHttpDate(
+						Instant.parse("2026-08-01T04:54:00Z")));
+	}
+
+	@Test
+	void preservesTwoDigitDayInIflytekSignatureDate() {
+		assertEquals(
+				"Tue, 11 Aug 2026 04:54:00 GMT",
+				IflytekScoringProvider.formatHttpDate(
+						Instant.parse("2026-08-11T04:54:00Z")));
+	}
+}