  We have incorporated time-dependent terms into the statistical and radiative
equilibrium calculations of the non-LTE line-blanketed radiative transfer code
CMFGEN. To illustrate the significant improvements in spectral fitting achieved
for photospheric phase Type II SN, and to document the effects associated with
time dependence, we model the outer 6.1Msun of ejecta of a BSG/RSG progenitor
star. Hopping by 3-day increments, we compute the UV to near-IR spectral
evolution for both continuum and lines, from the fully ionized conditions at
one week to the partially recombined conditions at 6 weeks after the explosion.
We confirm the importance of allowing for time-dependence in the modeling of
Type-II SN, as recently discussed by Utrobin & Chugai for SN1987A. However, we
allow in our approach for the full interaction between the radiation field and
level populations, and study the effects on the full spectrum. At the
recombination epoch, HI lines and NaD are stronger and broader than in
equivalent steady-state models, while CaII is weakened. Former successes of
steady-state CMFGEN models are unaffected, while former discrepancies are
cured. Time dependence affects all lines, while the continuum, from the UV to
the optical, changes only moderately. We identify two key effects: First, time
dependence together with the energy gain through changes in ionization and
excitation lead to an over-ionization in the vicinity of the photosphere,
dramatically affecting line optical depths and profiles. Second, the ionization
is frozen-in at large radii/velocities. This stems solely from the time-scale
contrast between recombination and expansion and will occur, modulo non-thermal
excitation effects, in all SN types. [abridged]
