  We numerically investigate attosecond streaking time delays in the
photoemission of valence and 2p core electrons of aluminum surface. We find
that electron emission from the core level band is delayed by $\Delta \tau
=100$ attoseconds relative to the release of electrons from the valence band.
We show that this relative time offset in electron emission is caused by the
screening of the streaking laser field by conduction electrons.
