The GIT_AUTHOR_DATE and GIT_COMMITTER_DATE environment variables[1] are what you use for things like that. (The former can also be overridden by the --date= option to git commit, and there’s also various situational stuff like --reset-author for git commit or --reset-author-date and --committer-date-is-author-date for git rebase. No, that last one is not parody.)

[1] https://git-scm.com/docs/git-commit#_commit_information