  Physical systems with material properties modulated in time provide versatile
routes for designing magnetless nonreciprocal devices. Traditionally,
nonreciprocity in such systems is achieved exploiting both temporal and spatial
modulations, which inevitably requires a series of time-modulated elements
distributed in space. In this paper, we introduce a concept of bianisotropic
time-modulated systems capable of nonreciprocal wave propagation at the
fundamental frequency and based on uniform, solely temporal material
modulations. In the absence of temporal modulations, the considered
bianisotropic systems are reciprocal. We theoretically explain the
nonreciprocal effect by analyzing wave propagation in an unbounded
bianisotropic time-modulated medium. The effect stems from temporal modulation
of spatial dispersion effects which to date were not taken into account in
previous studies based on the local-permittivity description. We propose a
circuit design of a bianisotropic metasurface that can provide
phase-insensitive isolation and unidirectional amplification.
