Add doc for timegm().
diff --git a/Doc/lib/libcalendar.tex b/Doc/lib/libcalendar.tex
index 0c7207b..59e3443 100644
--- a/Doc/lib/libcalendar.tex
+++ b/Doc/lib/libcalendar.tex
@@ -46,3 +46,11 @@
\begin{funcdesc}{prcal}{year}
Prints the calendar for the year \var{year}.
\end{funcdesc}
+
+\begin{funcdesc}{timegm}{tuple}
+An unrelated but handy function that takes a time tuple such are
+returned by the \function{gmtime()} function in the \module{time}
+module, and returns the corresponding Unix timestamp value, assuming
+an epoch of 1970, and the POSIX encoding. In fact,
+\function{gmtime()} and \function{timegm()} are each others inverse.
+\end{funcdesc}