diff --git a/src/libostree/ostree-core.h b/src/libostree/ostree-core.h
index b25112db..59d6d9e2 100644
--- a/src/libostree/ostree-core.h
+++ b/src/libostree/ostree-core.h
@@ -158,6 +158,17 @@ typedef enum {
  * - a(s(taya{sv})) - Map of ref name -> (latest commit size, latest commit checksum, additional metadata), sorted by ref name
  * - a{sv} - Additional metadata, at the current time the following are defined:
  *   - key: "ostree.static-deltas", value: a{sv}, static delta name -> 32 bytes of checksum
+ *   - key: "ostree.summary.last-modified", value: t, timestamp (seconds since
+ *     the Unix epoch in UTC, big-endian) when the summary was last regenerated
+ *     (similar to the HTTP `Last-Modified` header)
+ *   - key: "ostree.summary.expires", value: t, timestamp (seconds since the
+ *     Unix epoch in UTC, big-endian) after which the summary is considered
+ *     stale and should be re-downloaded if possible (similar to the HTTP
+ *     `Expires` header)
+ *
+ * The currently defined keys for the `a{sv}` of additional metadata for each commit are:
+ *  - key: `ostree.commit.timestamp`, value: `t`, timestamp (seconds since the
+ *    Unix epoch in UTC, big-endian) when the commit was committed
  */
 #define OSTREE_SUMMARY_GVARIANT_STRING "(a(s(taya{sv}))a{sv})"
 #define OSTREE_SUMMARY_GVARIANT_FORMAT G_VARIANT_TYPE (OSTREE_SUMMARY_GVARIANT_STRING)
diff --git a/src/libostree/ostree-repo-private.h b/src/libostree/ostree-repo-private.h
index 94330226..87e67a23 100644
--- a/src/libostree/ostree-repo-private.h
+++ b/src/libostree/ostree-repo-private.h
@@ -43,6 +43,14 @@ G_BEGIN_DECLS
  * */
 #define _OSTREE_MAX_OUTSTANDING_WRITE_REQUESTS 16
 
+/* Well-known keys for the additional metadata field in a summary file. */
+#define OSTREE_SUMMARY_LAST_MODIFIED "ostree.summary.last-modified"
+#define OSTREE_SUMMARY_EXPIRES "ostree.summary.expires"
+
+/* Well-known keys for the additional metadata field in a commit in a ref entry
+ * in a summary file. */
+#define OSTREE_COMMIT_TIMESTAMP "ostree.commit.timestamp"
+
 typedef enum {
   OSTREE_REPO_TEST_ERROR_PRE_COMMIT = (1 << 0)
 } OstreeRepoTestErrorFlags;
diff --git a/src/libostree/ostree-repo.c b/src/libostree/ostree-repo.c
index e204fd15..fd7aa55d 100644
--- a/src/libostree/ostree-repo.c
+++ b/src/libostree/ostree-repo.c
@@ -4563,6 +4563,10 @@ ostree_repo_verify_summary (OstreeRepo    *self,
  * An OSTree repository can contain a high level "summary" file that
  * describes the available branches and other metadata.
  *
+ * If the timetable for making commits and updating the summary file is fairly
+ * regular, setting the `ostree.summary.expires` key in @additional_metadata
+ * will aid clients in working out when to check for updates.
+ *
  * It is regenerated automatically after a commit if
  * `core/commit-update-summary` is set.
  */
@@ -4595,6 +4599,9 @@ ostree_repo_regenerate_summary (OstreeRepo     *self,
       const char *commit = g_hash_table_lookup (refs, ref);
       g_autofree char *remotename = NULL;
       g_autoptr(GVariant) commit_obj = NULL;
+      g_auto(GVariantDict) commit_metadata_builder = OT_VARIANT_BUILDER_INITIALIZER;
+      guint64 commit_timestamp;
+      g_autoptr(GDateTime) dt = NULL;
 
       g_assert (commit);
 
@@ -4608,11 +4615,21 @@ ostree_repo_regenerate_summary (OstreeRepo     *self,
       if (!ostree_repo_load_variant (self, OSTREE_OBJECT_TYPE_COMMIT, commit, &commit_obj, error))
         goto out;
 
-      g_variant_builder_add_value (refs_builder, 
+      g_variant_dict_init (&commit_metadata_builder, NULL);
+
+      /* Forward the commit’s timestamp if it’s valid. */
+      commit_timestamp = ostree_commit_get_timestamp (commit_obj);
+      dt = g_date_time_new_from_unix_utc (commit_timestamp);
+
+      if (dt != NULL)
+        g_variant_dict_insert_value (&commit_metadata_builder, OSTREE_COMMIT_TIMESTAMP,
+                                     g_variant_new_uint64 (GUINT64_TO_BE (commit_timestamp)));
+
+      g_variant_builder_add_value (refs_builder,
                                    g_variant_new ("(s(t@ay@a{sv}))", ref,
                                                   (guint64) g_variant_get_size (commit_obj),
                                                   ostree_checksum_to_bytes_v (commit),
-                                                  ot_gvariant_new_empty_string_dict ()));
+                                                  g_variant_dict_end (&commit_metadata_builder)));
     }
 
 
@@ -4661,6 +4678,11 @@ ostree_repo_regenerate_summary (OstreeRepo     *self,
     g_variant_dict_insert_value (&additional_metadata_builder, OSTREE_SUMMARY_STATIC_DELTAS, g_variant_dict_end (&deltas_builder));
   }
 
+  {
+    g_variant_dict_insert_value (&additional_metadata_builder, OSTREE_SUMMARY_LAST_MODIFIED,
+                                 g_variant_new_uint64 (GUINT64_TO_BE (g_get_real_time () / G_USEC_PER_SEC)));
+  }
+
   {
     g_autoptr(GVariantBuilder) summary_builder =
       g_variant_builder_new (OSTREE_SUMMARY_GVARIANT_FORMAT);