How to move starting point of branch to an older commit

I'd like to rebase my branch onto an older commit to exclude code added by the newer commits.

What I have:

A---B---C---D---E master
                 \
                  F---G---H---I---J experiment

What I want:

A---B---C---D---E master
     \
      F---G---H---I---J experiment

Say I wanted to make my experiment branch on commit B, but didn't realize I made it on E. By the time I realize, I already made a bunch of commits and they include code changes I don't want (C, D, E).

How do I move the starting point of experiment to the older commit B?

I've read this post but haven't seen anything usable regarding moving the branch to an older commit.

Solution

Simple:

git rebase --onto B master experiment

That command is read like this: git rebase --onto new-base discard-revisions-in-this-history this-is-what-i-want-to-rebase.

Git actually doesn't mind if the base is newer or older than where you are (simple example that might be rather common: a backport). It's all about the changes that are introduced that are in each revision that will be rebased.