Formulas - Dates
Excel recognises dates whether you use slash ("/") or hyphen ("-"). However if you use full stop (".") Excel would assume the date as a text. Therefore you will not be able to customise the date styles or even perform calculations with the date. So dates should be 25/12/2004 or 25-12-2004 and not 25.12.2004 (a pretty common mistake).