https://github.com/vyos/vyos-build/blob/current/scripts/make-version-file
It creates a file with the 'build on' timestamp and then the build process installs packages, therefore package installation dates and the build time can't be compared anymore to find out the status of the image.
https://phabricator.vyos.net/T2836
https://phabricator.vyos.net/T2835