  Solids exposed to intense electric fields release electrons through
tunnelling. This fundamental quantum process lies at the heart of various
applications, ranging from high brightness electron sources in DC operation to
petahertz vacuum electronics in laser-driven operation. In the latter process,
the electron wavepacket undergoes semiclassical dynamics in the strong
oscillating laser field, similar to strong-field and attosecond physics in the
gas phase. There, the sub-cycle electron dynamics has been determined with a
stunning precision of tens of attoseconds, but at solids the quantum dynamics
including the emission time window has so far not been measured. Here we show
that two-colour modulation spectroscopy of backscattering electrons uncovers
the sub-optical-cycle strong-field emission dynamics from nanostructures, with
attosecond precision. In our experiment, photoelectron spectra of electrons
emitted from a sharp metallic tip are measured as function of the relative
phase between the two colours. Projecting the solution of the time-dependent
Schr\"odinger equation onto classical trajectories relates phase-dependent
signatures in the spectra to the emission dynamics and yield an emission
duration of $710\pm30$ attoseconds by matching the quantum model to the
experiment. Our results open the door to the quantitative timing and precise
active control of strong-field photoemission in solid state and other systems
and have direct ramifications for diverse fields such as ultrafast electron
sources, quantum degeneracy studies and sub-Poissonian electron beams,
nanoplasmonics and petahertz electronics.
