In your Watchlist, thtat's the last-edited date you're seeing. That is, it's not the date they were invited, it's the date you last modified the profile. Of course, this would usually be the same if you're inviting a genealogist to join WikiTree.
If you just send a new invite, that doesn't get recorded as a change on the profile, and therefore doesn't change the last-edit date. I suppose we could start recording reinvites as changes, but I'd worry about crowding the activity feeds.