The error message is being displayed because the publisher is using epubcheck 1.x. To fix this issue:
a)
- rename the .epub file to .zip
- open the archive
- locate the .opf file
- copy it to a different folder
- find the line with the date in it
- change it to
2012-09-07
- replace the .opf file with the updated version
- rename .zip file back to .epub
b)
- open the .epub with
Sigil
- Prese F8 and add/modify the creation date
- save the .epub