What does =YEAR(C4) return (and similarly for the rest of column C)? Are
you sure that you haven't got text, rather than dates, in column C? If you
have, Excel will often translate them to date if they are in the format
expected by your regional settings, but if (for example) your windows
regional settings are looking for mm/dd/yyyy, you'll probably get #VALUE!
responses if you've got dd/mm/yyyy text entries. Similarly if you have a
non-date entry, you'll get #VALUE!