  We present a formalism to extract the EoR HI power spectrum for drift scans
using radio interferometers. Our main aim is to determine the coherence time
scale of time-ordered visibilities. We compute the two-point correlation
function of the HI visibilities measured at different times to address this
question. We determine, for a given baseline, the decorrelation of the
amplitude and the phase of this complex function. Our analysis uses primary
beams of four ongoing and future interferometers---PAPER, MWA, HERA, and
SKA1-Low. We identify physical processes responsible for the decorrelation of
the HI signal and isolate their impact by making suitable analytic
approximations. The decorrelation time scale of the amplitude of the
correlation function lies in the range of 2--20~minutes for baselines of
interest for the extraction of the HI signal. The phase of the correlation
function can be made small after scaling out an appropriate term, which also
causes the coherence time scale of the phase to be longer than the amplitude of
the correlation function. We find that our results are insensitive to the input
HI power spectrum and therefore they are directly applicable to the analysis of
the drift scan data. We also apply our formalism to a set of point sources and
statistically homogeneous diffuse correlated foregrounds. We find that point
sources decorrelate on a time scale much shorter than the HI signal. This
provides a novel mechanism to partially mitigate the foregrounds in a drift
scan.
