I'm trying to figure that out myself... I think this is it: Ela tem um carro. (Sing.) Eles têm um carro. (Plural) http://br.answers.yahoo.com/question/index?qid=20080209133105AAzNnjm
Except, "carta" is the Spanish word, not the Portuguese word for "map" so it doesn't work.
"Zero" is the same as "not any" or "no" which takes on plural.
- I do not have a shoe
- I do not have any shoes
- I have zero shoes
- I have no shoes
- There are not any shoes here
If you expect the letter to have a single date (most likely the date it was written), you would use "no date".
If you expect the letter to contain a few dates in its text, perhaps describing events, then you would use "no dates".
Now, since we have more than one letter, we expect at least one date for each, so the plural version is fine.