diff --git a/notmuch-client.h b/notmuch-client.h
index 1081a159..1e2a016a 100644
--- a/notmuch-client.h
+++ b/notmuch-client.h
@@ -124,7 +124,7 @@ int
 notmuch_tag_command (void *ctx, int argc, char *argv[]);
 
 const char *
-notmuch_time_relative_date (void *ctx, time_t then);
+notmuch_time_relative_date (const void *ctx, time_t then);
 
 void
 notmuch_time_print_formatted_seconds (double seconds);
diff --git a/notmuch-time.c b/notmuch-time.c
index de56db9e..e250c3d5 100644
--- a/notmuch-time.c
+++ b/notmuch-time.c
@@ -43,7 +43,7 @@
 #define DAY (24 * HOUR)
 #define RELATIVE_DATE_MAX 20
 const char *
-notmuch_time_relative_date (void *ctx, time_t then)
+notmuch_time_relative_date (const void *ctx, time_t then)
 {
     struct tm tm_now, tm_then;
     time_t now = time(NULL);