-to use universal time with @code{(setenv "TZ" "UTC0")}. If @env{TZ}
-is not in the environment, Emacs uses a platform-dependent default
-time zone.
+to default to universal time with @code{(setenv "TZ" "UTC0")}. If
+@env{TZ} is not in the environment, Emacs uses system wall clock time,
+which is a platform-dependent default time zone.
+
+@cindex time zone rule
+Functions that convert to and from local time accept an optional
+@dfn{time zone rule} argument, which specifies the conversion's time
+zone and daylight saving time history. If the time zone rule is
+omitted or @code{nil}, the conversion uses Emacs's default time zone.
+If it is @code{t}, the conversion uses Universal Time. If it is
+@code{wall}, the conversion uses the system wall clock time. If it is
+a string, the conversion uses the time zone rule equivalent to setting
+@env{TZ} to that string.