GIT_AUTHOR_DATE='Wed Dec 19 15:14:05 2029 -0800' git commit -m 'future!'
GIT_AUTHOR_DATE='2029-12-19 15:14:05 -0800' git commit -m 'future!'
GIT_AUTHOR_DATE='`date -Iseconds -u -R "oldest file"`' git commit -m 'past!'

To rewrite the repository to look as if foodir/ had been its project root, and discard all other history:

git filter-branch --subdirectory-filter ABC --prune-empty -- --all

Remove directory from history:

git filter-branch --index-filter "git rm -q -r -f --cached --ignore-unmatch ABC" --prune-empty -- --all

Mit Subtree:

## create branch that tracks only MyLibraryRepo/*
 
git subtree split -P MyLibraryRepo -b export
 
## make new repo
cd ~/Projects
mkdir LibraryRepo
cd LibraryRepo
git init
# copy over commits... 
git fetch ../bigrepo export
# ... and define master as them
git checkout -b master FETCH_HEAD

git log --format="- %B" --reverse Previous-Release..HEAD