Metasurfaces are artificially engineered two-dimensional materials composed of sub-wavelength meta-atoms, which have shown unprecedented capabilities in manipulating the amplitude, phase, frequency, and polarization states of electromagnetic waves. Specifically, polarization control can be attained via suitable anisotropic, linear, and time-invariant designs, while frequency conversion is realized via nonlinear or time-varying platforms. Simultaneous manipulations of polarization and frequency would be of considerable practical interest in many application scenarios, but remain unattainable with current approaches. Here, a time-division-multiplexing metasurface is proposed to realize the simultaneous conversion of polarization and frequency. The platform relies on time-modulated polarization switches and, by varying the duty cycle and time delays of the polarization channels, can arbitrarily rotate the polarization at the central frequency of operation, and synthesize various polarization states at selected harmonic frequencies. Theoretical predictions are validated via measurements on a prototype operating at microwave frequencies, providing the first experimental evidence of simultaneous polarization and frequency conversions via time-division-multiplexing metasurfaces. The outcomes open a new pathway in manipulating the electromagnetic waves via time-varying metasurfaces, and may be of interest for a broad variety of applications in scenarios ranging from polarization imaging to quantum optics.