cvs: no tag for merge was created - go back by date?

[KM]

I was looking at the online manual.  I don't see a merge tag for the branch 
around the date of the last merge, March 2011.   I was not working here at the 
time, but we need to merge it into the head.
 
How can I create a retroactive merge tag for 3/5/11 on my branch,  to use as 
the start tag?  I can of course create a current merge tag for now.
 
Also I am using eclipse to merge - but that should not matter. 
 
Very confusing... thx
KM