implemented proposal by niksnut.

Now you have to use either date= or tag= when specifying cvs revision

svn path=/nixpkgs/trunk/; revision=9661

pkgs/build-support/fetchcvs/builder.sh

@@ -1,8 +1,5 @@
 source $stdenv/setup
 
-if test -z "$tag"; then
-  tag="-DNOW"
-fi
 # creating the export drictory and checking out there only to be able to
 # move the content without the root directory into $out ...
 # cvs -f -d "$url" export $tag -d "$out" "$module"
@@ -10,6 +7,16 @@ fi
 # See als man Page for those options
 
 ensureDir $out export
+set -x
+if [ -n "$tag" ]; then
+  tag="-r $tag"
+else
+  if [ -n "$date" ]; then
+    tag="-D $date"
+  else
+    tag="-D NOW"
+  fi
+fi
 cd export; cvs -f -d "$url" export $tag "$module"
 mv */* $out