A family of Microsoft word processing software products for creating web, email, and print documents.
Format the date in your letter in a character style called Date. Use a StyleRef field in the header to reflect that date.
Ideally you have a letter template which has a CreateDate field in it in the body and a header set up to show that date on the second and subsequent page using a StyleRef field.
That way when you write the letter the date will be the date on which the letter is created. It will stay that date even if you open the letter later to print another copy. You can type over it and change it if needed. The date in the header will be whatever is typed in the Date style in the body of the letter.
You may want to look at the tutorial on StyleRef fields and letters. It has samples.