Unattended_install will install as long as there are no BLOCKING applications running -- sometimes this will be just the app itself, other times it can be a suite. 
The date/time to immediately install isn't quite correct: managed software update will give a user warnings that it has to be installed by X date and time and it will get progressively more annoying as the time approaches (I _am_ a bad user and I ignore my own updates).  IF you use munki as your initial software load, or add a rogue/new mac user to your munki server and there's a pkginfo file with a force install by date in the past, it will give a countdown period (an hour?)
This page covers both the intricacies of unattended installs and force install AFTER date (scroll down a bit, they're right next to each other)