Current implementation (as of 3.0.2) does not recognize correctly date formats containing format specifiers in upper case, nor it recognizes dot as a date separator and time format specifiers as a valid datetime format. Attached patch adds these format specifiers to the regex. Change: "^[ymd\\-/, ]+$" => "^[yYmMdDhHsS\\-/,. :]+$"
Created attachment 21491 [details] patch for HSSFDateUtil.isADateFormat()
Thanks for this patch, applied to trunk (along with a test)