How to git blame to see what code from a single committer before X date has survived?

How to git blame to see what code from a single committer before X date
has survived?

I joined a very experienced developer team as a complete RoR newb a year
ago and now we're trying to guess how much of the code I wrote in the
first six months has survived.
I think I can use git blame on the entire repo and then grep for my
username but I'm hitting a wall.