Then launch the installer normally. Once installation completes, re-sync your date automatically via System Preferences > Date & Time.
The most reliable method to get the official, unmodified full installer is via Apple’s support website. download macos 1012 sierra full
The full version of macOS 10.12 Sierra can be downloaded from the Mac App Store or through other reliable sources. Here's a step-by-step guide: Then launch the installer normally