While exporting my ePub document from my InDesign file, I had specified the date in the MetaData tab as: 28/12/2014
When running the resulting file through epubchecker, I received a warning about the date format being incorrect:
"WARNING: /Applications/ePubs/test.epub/OEBPS/content.opf(12,32): date value '28/12/2014' does not follow recommended syntax as per http://www.w3.org/TR/NOTE-datetime: [For input string: "28/12/2014"] is not an integer"
Checking the URL, dates should be formatted as YYYY-MM-DD, etc...
So, in the ePub export panel, I specified the date as 2014-12-28 and that is what is entered in the content.opf file and the warning is fixed.
Would be nice to see this handled properly in the export panel, but thought this would be useful for others.