Each date system uses a unique starting date from which all other workbook dates are calculated. LessĮxcel supports two date systems, the 1900 date system and the 1904 date system. Excel for Microsoft 365 Excel for Microsoft 365 for Mac Excel 2021 Excel 2021 for Mac Excel 2019 Excel 2019 for Mac Excel 2016 Excel 2016 for Mac Excel 2013 Excel 2010 Excel 2007 Excel for Mac 2011 More.