"cvs rtag -D" not working with removed files?

Hello there,

I want to use "cvs rtag -D" to create a tag based by a certain date.

Example:
  cvs rtag -D '2008-12-01 10:00' branch_name module_name

Unfortunately that does not tag files that have been removed in HEAD 
revision. Nevertheless, those files _did_ exist at the time in the -D 
option and were still available in the attic. 

Strangely enough, those "untaggable" files can be checked out using the 
same -D option with command "checkout":

  cvs co -D '2008-12-01 10:00' module_name

I wonder why "rtag -D" does not work as I expect it to since it seems to 
me that it should work, which means that I'm either misunderstanding 
something basic or that something else is wrong.

Thanks.

Daniel

PS: using cvs version 1.12.13