Everything downloaded from the Mac App Store is signed with Apple’s worldwide developer certificate to ensure it is valid. This certificate expired for most App Store listings on 2/13/2016, and was auto-renewed by Apple on their backend.
This certificate expiration/renewal is not something that will affect most Mac users. If however you have previously downloaded an installer from the Mac App Store, and retained a copy of it for later use, there’s a good chance these won’t work anymore. Attempting to run an installer with an outdated certificate will result in the error “This copy of the [app name] application can’t be verified. It may have been corrupted or tampered with during download.”
It’s common practice for Mac support engineers to keep copies of installers/updates on an external drive, and reuse them over and over the Macs they service. Outdated copies will need to be replaced by signing into the Mac App store with an Apple ID that previously downloaded the installer, then visit the Purchases tab (even for free installers).
Re-downloading older OS X installers can be challenging because the App Store will only show items compatible with the Mac you are logging in with (the Mavericks install is only available on Macs manufactured in Fall 2013 or older). I have confirmed that a mid-2010 Mac mini will show the following OS X installers available for re-download: 10.9, 10.10, 10.11. I also tested my older OS X installers (10.6,10.7, 10.8) and they all worked as-is, as I’m presuming they predate using an Apple worldwide developer certificate.