@talendtester: right, Excel automatically converts date fields to a default format. But the input or output of Excel would still be the actual value ("2012" but shown as "01-01-2012" in Excel).
By the way same issue causes for strings and numbers, e.g. you output a number as string then excel could not calculate with it. So you better take care of formats in Talend to garantee in Excel works everything fine.