Install a package with an expired certificate

I just noticed I can double-clik, open and install a .pkg package installer that has an expired certificate **without any popup** like *"This package is from an developer whose certificate has expired"* or *"This package has expired"*. Instead there is no alert at all. If I click on the top right "Lock" symbol, in the details of the certificate, I see a green tick mark with *"This certificate is valid"*. Is it the normal behaviour? Is it true on all recent Mac OS X version that I can install old installers from a few years ago (for which the developer certificate has expired), **without having an alarming popup**? I have myself a few old .pkg installers (the program itself has not been updated), and I don't know if it's mandatory for me to rebuild the installer with my new certificate, or if everyone can still install the old installer (with expired certificate) without any alarm.