You don't. Well if you mean actually delete all the files you have worked on today then you can do that.
The "today" search in Finder searches the files indexed by spotlight for files that have been modified/created today. Those are the actual files listed.
The same goes for the spotlight history. You can however disable spotlight from indexing anything however this will stop all spotlight searches from working.
This command should disable searching for files:
Code:
sudo mdutil -a -i off
and this should re-enable it:
Code:
sudo mdutil -a -i on
If you just don't want the searches appearing in the Finder window, then you can drag them off to get rid of them. Or you can go to Finder > Preferences. Then on the sidebar tab you can uncheck the default searches you don't want appearing.