Fixed #27225 -- Added "Age" header when fetching cached responses.

Co-Authored-By: Author: Alexander Lazarević <laza@e11bits.com>

django/middleware/cache.py

@@ -43,6 +43,8 @@ More details about how the caching works:
 
 """
 
+import time
+
 from django.conf import settings
 from django.core.cache import DEFAULT_CACHE_ALIAS, caches
 from django.utils.cache import (
@@ -53,6 +55,7 @@ from django.utils.cache import (
     patch_response_headers,
 )
 from django.utils.deprecation import MiddlewareMixin
+from django.utils.http import parse_http_date_safe
 
 
 class UpdateCacheMiddleware(MiddlewareMixin):
@@ -171,6 +174,15 @@ class FetchFromCacheMiddleware(MiddlewareMixin):
             request._cache_update_cache = True
             return None  # No cache information available, need to rebuild.
 
+        # Derive the age estimation of the cached response.
+        if (max_age_seconds := get_max_age(response)) is not None and (
+            expires_timestamp := parse_http_date_safe(response["Expires"])
+        ) is not None:
+            now_timestamp = int(time.time())
+            remaining_seconds = expires_timestamp - now_timestamp
+            # Use Age: 0 if local clock got turned back.
+            response["Age"] = max(0, max_age_seconds - remaining_seconds)
+
         # hit, return cached response
         request._cache_update_cache = False
         return response