indicate that mktime is platform-dependent. See SF bug 434143.
diff --git a/Doc/lib/libtime.tex b/Doc/lib/libtime.tex
index 2e9de60..d28e863 100644
--- a/Doc/lib/libtime.tex
+++ b/Doc/lib/libtime.tex
@@ -173,7 +173,8 @@
the dst flag if it is unknown) which expresses the time in
\emph{local} time, not UTC. It returns a floating point number, for
compatibility with \function{time()}. If the input value cannot be
-represented as a valid time, \exception{OverflowError} is raised.
+represented as a valid time, \exception{OverflowError} is raised. The
+earliest date for which it can generate a time is platform-dependent.
\end{funcdesc}
\begin{funcdesc}{sleep}{secs}