I'm writing a script to serve as an example of Git usage. However, every time I run it, a different set of hashes are produced, even if the commit message, author, date, parent and contents are the same:

```
rm -rf /tmp/git-teste
git init /tmp/git-teste
cd /tmp/git-teste
echo 'zero' > master.txt
git add .
git commit -am zero --date '2013-05-28 16:40:00' --author 'andre <teste@teste.com>'

```

Shouldn't the hash has been always the same? What should I change to assure this happen?