When did Saint Stephen live?

Saint Stephen is only known from Acts of the Apostles, which merely describes him as a deacon of the early Church, who was arrested and stoned for blasphemy. This would suggest that he died in the mid-30s of the first century. However, questions have been raised about the historicity of Stephen. Raymond E. Brown (An Introduction to the New Testament) says that we can never verify the martyrdom or even existence of Stephen.