According to my sources, it was Winter Update 2005 that Cipsoft implemented where you can see who wrote on a document, letter, label, book, etc. However, not sure when they added the dates to the letters as well.
My husband has a label that doesn't show the date it was written on but has a name. I know his account and both the characters were created on July 19, 2004-
Here is the oldest letter I could find in my depot dated July 21, 2008, that has the name and date on the letter-
Sources:
News: Dec 12 2005- Game Update 7.6
"In addition to these major changes, a number of minor adjustments have been made. For example, letters, labels and other editable items now show the name of the most recent editing character."
Related Question: Is there a way to know who sent you a letter from before the senders name was on it?