Edit: I found that my statements below are incorrect. Everything exported to Excel is exported as a text. At some point in time, the cells were correctly typed but that was not was some people expected because the output would not be exactly the same as in the data grid.
I've created a support ticket myself to also add the 'Use grid date format' to the Export-to-Excel button. Furthermore, this setting needs to respect the fact whether the column's date format is date, time or date&time. We will fix both issues in 2.5.8.
If you export a column with an attribute of type DateTime to Excel the cell is marked as a date and the value can be used as a date.
If you export a column with an attribute of type String the cells are marked as texts and Excel will not try to see a date in it.
When exporting to CSV, Excel will try to see dates in texts. 2.5.7 will introduce a feature that allows you to export a date using a standard format that will always be correctly recognized by Excel (year-month-day).
We work with Mendix 3.2.1 but the issue described by Frans Verschoor still exists on our end. I also don't have the option to export dates using a standard date format like I have for the CSV export button. Any clue what could be wrong?