.xlsx Dates are internally represented as numbers. If you add Format of your Date column in Excel or Google Spreadsheet, you will see that the value is exported correctly. We put a default ‘d’ format in the ver. 4.4.0 for Dates.
.html Dates export depends on the Grid’s cellsFormat property. The default value is ‘d’ which is (dd/MM/YYYY) during the Grid export, when no format is set. If you want to keep the time component, you will need to set your columns format.