indicate that mktime is platform-dependent. See SF bug 434143.

Doc/lib/libtime.tex

@@ -173,7 +173,8 @@ is the full 9-tuple (since the dst flag is needed; use \code{-1} as
 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}