https://stackoverflow.com/questions/11856983/why-git-authordate-is-different-from-commitdate
The author date notes when this commit was originally made (i.e. when you finished the git commit). The author date could be overridden using the --date switch.
The commit date gets changed every time the commit is being modified, for example when rebasing the branch.
Github displays commits based on commit date.
So, after a rebase, your commits timeline will be messed up on Github, though in reality, by git log
you can see your author date is intact.
You may want to do this to rectify the GitHub's commits timeline.
https://stackoverflow.com/questions/28536980/git-change-commit-date-to-author-date
git filter-branch --env-filter 'export GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"'