Retrieve an older version of a git repository

Hello,

I have a problem with a java source and I’d like to retrieve older version of it.
I don’t know how to do. I know approximately the date of modification, that’s all.

Assuming you already have the repository locally: You can use git log to check the commit history, and find the one you want, see Git - Viewing the Commit History. After that you can use git checkout on the commit to get the actual source. If you want to figure out exactly which commit introduced a bug git bisect might be helpful, see Git - Debugging with Git.