My answer assumes this works on a MAC the same way it does in Windows. Try one or two to be sure before going wholesale. Press refresh after trying the first couple to have them removed from your list.
First gather all those files in a list...
one way to do this is to open a view that has all of your audio files and then, in the search bar up there in the top right, paste this:
[=isequal([date],//,7)]=1
Confirm that the results are those you want, and select all files in the list.
Open the tag window. (lower left corner)
The Date field will show "[Varies]" (without quotes), click to open it for editing, and paste this in there:
=removeleft([date],6)
and press enter.
Job (should be) done.
If you're not happy, press ctrl+z to undo.
I say "should be" done because dates can be tricky with different locales and so on, but doing this way should get you the results you want reagardless of all that.
-marko