On their search details page github lists some ways to find this information:
https://help.github.com/articles/searching-repositories
You can filter repositories based on times of creation, or when they were last updated. For repository creation, you can use the created qualifier;
Example:
ruby created:>2014-01-01
Matches repositories with the word "ruby" that were created after Jan 1, 2014
Good luck, and I'm glad you are wanting to contribute to an open source project!