It is not possible to change the date of a commit in Git, not without massive hacking. However, it is possible to rewrite a new commit in place of an old commit. You might be able to rewrite a certain commit at a certain time. An interactive rebase and filter-branch are two ways that the history of a branch can be rewritten. But in both cases, it will be very visible that one or more commits have be rewritten, by you.
In general, you should avoid rewriting history if possible. This answer assumed that the branch in question has already been shared publicly, i.e. other users have seen your branch. If your branch was never pushed, then technically you can rewrite it all you want, and no one will be able to tell a rewritten commit from the original; they would just see a bunch of commits on the remote.