Created attachment 29504 [details] the sample file org.apache.poi.ss.usermodel.DateUtil.isCellDateFormatted can't work right when excel2007 cell format is "yyyy-MM-ddTHH:mm:ss". The method return false that is not right,because we call getDateCellValue() on the Cell object,it return the right java.util.Date. Use the Book1.xlsx to reproduce it.
please also check org.apache.poi.ss.usermodel.DataFormatter.formatRawCellContents for format string "yyyy-MM-ddTHH:mm:ss". it can't process it right.
Are you able to create a unit test that shows the problem? (problems?) And does it work for other simpler formats?
Created attachment 29505 [details] testcase
it work for other simpler formats.
Thanks for the testcase for this, fixed in r1597038.