2 files changed, 7 insertions, 1 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 98a4f978830..95c8ce2cb51 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,8 @@
+2012-07-08  Paul Eggert  <eggert@cs.ucla.edu>
+
+        * systime.h (EMACS_SUB_TIME): Clarify behavior with unsigned time_t.
+        See <http://bugs.gnu.org/11825#29>.
+
 2012-07-08  Eli Zaretskii  <eliz@gnu.org>
 
         * xdisp.c (fill_glyphless_glyph_string): If the face of the glyph
diff --git a/src/systime.h b/src/systime.h
index 938083a5ffc..a03630cf936 100644
--- a/src/systime.h
+++ b/src/systime.h
@@ -80,7 +80,8 @@ typedef unsigned long Time;
 #define EMACS_GET_TIME(time)                gettime (&(time))
 
 /* Put into DEST the result of adding SRC1 to SRC2, or of subtracting
-   SRC2 from SRC1.  On overflow, store an extremal value.  */
+   SRC2 from SRC1.  On overflow, store an extremal value: ergo, if
+   time_t is unsigned, return 0 if the true answer would be negative.  */
 #define EMACS_ADD_TIME(dest, src1, src2) ((dest) = timespec_add (src1, src2))
 #define EMACS_SUB_TIME(dest, src1, src2) ((dest) = timespec_sub (src1, src2))