  We study laser-assisted photoionization by attosecond pulses using a
time-independent formalism based on diagrammatic many-body perturbation theory.
Our aim is to provide an ab inito route to the "delays" for this
above-threshold ionization process, which is essential for a quantitative
understanding of attosecond metrology. We present correction curves for
characterization schemes of attosecond pulses, such as "streaking", that
account for the delayed atomic response in ionization from neon and argon. We
also verify that photoelectron delays from many-electron atoms can be measured
using similar schemes if, instead, the so-called continuum--continuum delay is
subtracted. Our method is general and it can be extended also to more complex
systems and additional correlation effects can be introduced systematically.
