The certificate expired. Apple's old installer certificates have a limited lifespan. Fix: Disconnect from the internet and manually set the date back via Terminal before installing:
If you "purchased" (downloaded) Mavericks when it was originally released in 2013, it may still appear in your tab in the Mac App Store. Spiceworks Community mavericks os x 10.9.x dmg
OS X 10.9 Mavericks: Everything You Need to Know About the DMG and Installer The certificate expired
For users looking to maintain legacy hardware or perform a clean installation, the file is the essential starting point. Technical Overview of OS X Mavericks 10.9.x Spiceworks Community OS X 10
Have a tip on finding rare DMGs? Share in the comments below. For more legacy macOS guides, check out our series on Snow Leopard and Mountain Lion DMG files.
: Minimum 2 GB of RAM (4 GB is recommended for better performance). Storage : At least 8 GB of available space for installation. Compatible Models : iMac : Mid 2007 or newer. MacBook : Late 2008 Aluminum or Early 2009 Plastic or newer.