Created attachment 175743 [details] File to save as xlsx. Close, reopen, dates have changed compared to original When a .ods file is saved as .xlsx, the dates are incorrectly converted and come out two days earlier! (Open the uploaded file. Save it as xlsx. Close. Re-open. Dates have changed -2 days)
Have just established that it doesn't happen with a newly created .ods file. The file I have submitted that does convert to xlsx incorrectly is based on a template file that was generated many years ago. Some internal date representation version issue?
It is because you have set up as 01/01/1900 in Menu/Tools/Options/LibreOffice calc/Calculate - Date Choose the first option and it should work fine.