You clone source from git, but then you use them to build some artifacts. The artifacts build time may differ, yet with reproducible builds - the artifact should match.
Right, but if you only clone and build, why would the files modification date be different compared to the version that was committed to git? Does just cloning a repo already lead to different file modification dates in my local copy?
Git does not store or restore file modification times.
And the reason for that in turn is because if you are on one commit and check out and older commit, then restoring file modification times to what they were at the time of the older commit would cause build tools that look at file modification times to sometimes not pick up on all the changes.