Hi,
It's the standard functionality. You actually have to enhance the standard method, which is not advisable as the same will be called in other applications as well.
So, Instead of that, hide the standard Export functionality and create a new Tool bar button, say, 'Export to Excel' and in on action of that button, use cl_wd_runtime_services=>attach_file_to_response ( and pass the file name by concatenating date/time) method to open/save the file.
Check this document to create a custom button in ALV tool bar: Creating Self-Defined Functions in WDA ALV
And, check this blog by Chris for code reference to have Export to Excel functionality: Using WD ABAP ALV export - the hacked way
Hope this helps u,
Regards,
Kiran