Rule to find unused apps

I have a rule to move unused apps to another directory:

Date Last Opened iss before specific date 1/1/2012
Extension is app
Subfolder depth is less than 1

However some apps like Dropbox.app are being moved even though I have just used it today. One weird thing I noticed is that if I use the command line tool mdls it does not have the kMDItemLastUsedDate attribute like some of my other apps do. In finder Dropbox.app shows a last opened of November 27,2012 which is the same date as the Modified Date even though I opened it today.

Dropbox.app was one example, it happens to other apps as well. I think the key is that not all of my apps have the meta data item kMDItemLastUsedDate. Which is weird, any thoughts?

Hmmmm... A quick look at my Apps folder also shows that some apps I use every day do not have a properly set kMDItemLastUsedDate. For me, there seems to be a correlation between UI apps and background apps (e.g. Dropbox, Alfred are not set but Word, Xcode are). I agree - this seems to be a mystery.

On a side note, I very much like the idea of deleting unused apps. Great idea!

I think it might be a case of apps launched by the system vs apps explicitly launched by the user via Finder, with the latter getting the date updated properly. I haven't really played with it recently but seem to recall something along these lines.