Fix SF #618146, overflow error in calendar module
Will backport.
diff --git a/Doc/lib/libcalendar.tex b/Doc/lib/libcalendar.tex
index 57ed3b0..222e2db 100644
--- a/Doc/lib/libcalendar.tex
+++ b/Doc/lib/libcalendar.tex
@@ -15,6 +15,11 @@
week to Sunday (6) or to any other weekday. Parameters that specify
dates are given as integers.
+Most of these functions rely on the platform provided \function{mktime()}.
+Therefore, valid argument values may vary from system to system.
+On Unix, valid years are typically between \code{1970} and \code{2037},
+but may be work between \code{1902} and \code{2037}.
+
\begin{funcdesc}{setfirstweekday}{weekday}
Sets the weekday (\code{0} is Monday, \code{6} is Sunday) to start
each week. The values \constant{MONDAY}, \constant{TUESDAY},