Comment 2 for bug 851280

This seems to be caused by the lazy loading behavior of the package history: when the get_installed_date is first called,the apt history is not initialized yet, thus it will return None (and we don't display that), and adds an idle callback to rescan the history, which will properly initialize it, that is why subsequent calls work as expected.
I'm not sure what we should do here, as it's either performance (lazy loading helps) or consistency (on first load, we won't have the installed date), I guess this needs a decision to be taken, unless someone finds a third alternative.