diff --git a/database.cc b/database.cc
index 928e91ba..aaad7105 100644
--- a/database.cc
+++ b/database.cc
@@ -588,7 +588,8 @@ notmuch_database_set_timestamp (notmuch_database_t *notmuch,
     try {
 	status = find_timestamp_document (notmuch, db_key, &doc, &doc_id);
 
-	doc.add_value (0, Xapian::sortable_serialise (timestamp));
+	doc.add_value (NOTMUCH_VALUE_TIMESTAMP,
+		       Xapian::sortable_serialise (timestamp));
 
 	if (status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND) {
 	    char *term = talloc_asprintf (NULL, "%s%s",
@@ -630,7 +631,7 @@ notmuch_database_get_timestamp (notmuch_database_t *notmuch, const char *key)
 	if (status == NOTMUCH_PRIVATE_STATUS_NO_DOCUMENT_FOUND)
 	    goto DONE;
 
-	ret =  Xapian::sortable_unserialise (doc.get_value (0));
+	ret =  Xapian::sortable_unserialise (doc.get_value (NOTMUCH_VALUE_TIMESTAMP));
     } catch (Xapian::Error &error) {
 	goto DONE;
     }
diff --git a/message.cc b/message.cc
index 78b6fed6..f2405ac6 100644
--- a/message.cc
+++ b/message.cc
@@ -268,7 +268,7 @@ _notmuch_message_set_date (notmuch_message_t *message,
 
     time_value = notmuch_parse_date (date, NULL);
 
-    message->doc.add_value (NOTMUCH_VALUE_DATE,
+    message->doc.add_value (NOTMUCH_VALUE_TIMESTAMP,
 			    Xapian::sortable_serialise (time_value));
 }
 
diff --git a/notmuch-private.h b/notmuch-private.h
index 88b01bd3..53e99bdd 100644
--- a/notmuch-private.h
+++ b/notmuch-private.h
@@ -67,13 +67,10 @@ NOTMUCH_BEGIN_DECLS
 #endif
 #endif
 
-/* These value numbers are chosen to be sup compatible (for now at
- * least). */
-
 typedef enum {
-    NOTMUCH_VALUE_MESSAGE_ID = 0,
-    NOTMUCH_VALUE_THREAD = 1,
-    NOTMUCH_VALUE_DATE = 2
+    NOTMUCH_VALUE_TIMESTAMP = 0,
+    NOTMUCH_VALUE_MESSAGE_ID,
+    NOTMUCH_VALUE_THREAD
 } notmuch_value_t;
 
 /* Xapian (with flint backend) complains if we provide a term longer
diff --git a/query.cc b/query.cc
index ae986d35..c68bd37a 100644
--- a/query.cc
+++ b/query.cc
@@ -114,10 +114,10 @@ notmuch_query_search (notmuch_query_t *query)
 
 	switch (query->sort) {
 	case NOTMUCH_SORT_DATE_OLDEST_FIRST:
-	    enquire.set_sort_by_value (NOTMUCH_VALUE_DATE, FALSE);
+	    enquire.set_sort_by_value (NOTMUCH_VALUE_TIMESTAMP, FALSE);
 	    break;
 	case NOTMUCH_SORT_DATE_NEWEST_FIRST:
-	    enquire.set_sort_by_value (NOTMUCH_VALUE_DATE, TRUE);
+	    enquire.set_sort_by_value (NOTMUCH_VALUE_TIMESTAMP, TRUE);
 	    break;
 	case NOTMUCH_SORT_MESSAGE_ID:
 	    enquire.set_sort_by_value (NOTMUCH_VALUE_MESSAGE_ID, FALSE);