Haven't done so myself - But you can edit a git commit's information (Such as author and date):

http://stackoverflow.com/questions/454734/how-can-one-change...

Edit: Haven't looked at the repo - But I'm assuming this is what's being done.

It's a Python script that takes in some options (which image, your github username, etc) and generates a bash script. Running the bash script actually makes the commits on the correct days to populate the pixels.

luckily git takes both GIT_AUTHOR_DATE and GIT_COMMITTER_DATE as environment variables. This python script generates a bash script that sets a date and commit one line changes for every pixel that needs some gitfiti.