Change description of altzone to not refer to the "0th meridian".

Doc/lib/libtime.tex

@@ -111,8 +111,8 @@ string.  It may also be modified at run time.
 
 \begin{datadesc}{altzone}
 The offset of the local DST timezone, in seconds west of UTC, if one
-is defined.  Negative if the local DST timezone is east of the 0th
-meridian (as in Western Europe, including the UK).  Only use this if
+is defined.  This is negative if the local DST timezone is east of UTC
+(as in Western Europe, including the UK).  Only use this if
 \code{daylight} is nonzero.
 \end{datadesc}