diff --git a/httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/ResponseCacheConformance.java b/httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/ResponseCacheConformance.java
index 87ef6857e3..37a368fbff 100644
--- a/httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/ResponseCacheConformance.java
+++ b/httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/ResponseCacheConformance.java
@@ -29,6 +29,7 @@
 import java.io.IOException;
 import java.time.Instant;
 
+import org.apache.hc.client5.http.utils.DateUtils;
 import org.apache.hc.core5.annotation.Contract;
 import org.apache.hc.core5.annotation.ThreadingBehavior;
 import org.apache.hc.core5.http.EntityDetails;
@@ -71,7 +72,7 @@ public void process(final HttpResponse response,
             }
         }
         if (!response.containsHeader(HttpHeaders.DATE)) {
-            response.addHeader(new BasicHeader(HttpHeaders.DATE, Instant.now()));
+            response.addHeader(new BasicHeader(HttpHeaders.DATE, DateUtils.formatStandardDate(Instant.now())));
         }
     }
 
diff --git a/httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestResponseCacheConformance.java b/httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestResponseCacheConformance.java
index df347b7b8a..1983376265 100644
--- a/httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestResponseCacheConformance.java
+++ b/httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestResponseCacheConformance.java
@@ -31,9 +31,11 @@
 import java.time.Instant;
 
 import org.apache.hc.client5.http.utils.DateUtils;
+import org.apache.hc.core5.http.HttpHeaders;
 import org.apache.hc.core5.http.HttpResponse;
 import org.apache.hc.core5.http.HttpStatus;
 import org.apache.hc.core5.http.support.BasicResponseBuilder;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -87,4 +89,14 @@ void shouldStripContentTypeFromOrigin304ResponseToStrongValidation() throws Exce
                 "Content-Type", "text/html;charset=utf-8");
     }
 
+    @Test
+    void shouldAddStandardDateHeaderIfMissing() throws Exception {
+        final HttpResponse response = BasicResponseBuilder.create(HttpStatus.SC_OK)
+                .build();
+        assertFalse(response.containsHeader(HttpHeaders.DATE));
+        impl.process(response, null, null);
+        Assertions.assertTrue(response.containsHeader(HttpHeaders.DATE));
+        Assertions.assertNotNull(DateUtils.parseStandardDate(response, HttpHeaders.DATE));
+    }
+
 }