"Date Added" is the same as "[Date Imported]".
If [date imported] has taken the date you re-installed MC, This means that you have a whole new library?
Check your JRiver folder in Documents to see if you still have your library backups. Are they there? If they are, you could restore from the latest backup and then you would have your [date imported] data back.
If you removed MC and began again because you felt your library was out of control, we could probably help with some library maintainence stuff that would allow you to continue with your original library.
EDIT:OK, apologies, I see that was covered already. Still, do you have your backups?
If you do, make a backup of your current library and close MC.
Next, locate the latest library backup you have. It will be a zip file. Open it.
Inside, you will find a file named "field (date imported).jmd"
Take that file from the backup and add it to your library folder, replacing the existing file
Start MC. The file from the backup will not contain any data for file imported since the backup was made, which could prevent this trick from working, but still might be worth a punt.
Did it work?
If it did, the "Woohoo", if not, restore from the backup you made earlier and ignore me
-marko