The substring function returns a specific part of a text string. The first argument defines the text string, the second argument defines the starting position, and the third argument defines how many characters to include.
So if the function today() returns a value of 2014-02-04, the value for substring(today(), 0,8) now will be:
2014-02
Where 0 is the starting position, and 8 displays the number of characters. The formula is also equal to substring(today(), 1,7) since 0 does not really exist.
Going back to the issue, the problem here is specifying the file name at the submit data connection wizard. So using a field from your form which generates the file name instead of the formula substring(today(), 0,8) should enable you to specify which ever file name you desire.
After configuring this set-up, drag the field on to your canvas. Preview your form, enter the desired file name, update and submit your form. Hope this helps.