Is there any way to know or get the original create/modified timestamps? Thanks.
NO, Git simply does not store such (meta-)information, unless you use third party tools like metastore or git-cache-meta. The only timestamp that get stored is the time patch (author time) and commit (committer time) was created.
That is by design, as Git is version control system, not a backup utility or synchronization tool.
I believe that the only timestamps recorded in the Git database are the author and commit timestamps. I don't see an option for Git to modify the file's timestamp to match the most recent commit, and it makes sense that this wouldn't be the default behavior (because if it were, Makefiles wouldn't work correctly).
You could write a script to set the modification date of your files to the the time of the most recent commit. It might look something like this:
for FILE in $(git ls-files)
do
TIME=$(git log --pretty=format:%cd -n 1 --date=iso $FILE)
TIME=$(date -j -f '%Y-%m-%d %H:%M:%S %z' "$TIME" +%Y%m%d%H%M.%S)
touch -m -t $TIME $FILE
done