<p>Terahertz time-domain spectroscopy (THz-TDS) is widely used to determine the complex conductivity of thin films from transmission measurements. However, for ultrathin films with thicknesses much smaller than the THz wavelength, the retrieved conductivity becomes highly sensitive to small phase uncertainties in the measured transmission spectrum. In this work, we systematically investigate the impact of sub-picosecond timing errors on complex conductivity extraction in THz-TDS. Transmission measurements were performed on 400-nm-thick FAPbI₃ (formamidinium lead iodide) films coated on dielectric substrates, and the spectra were analyzed using a Lorentz oscillator model. Small temporal offsets between the sample and reference waveforms introduce systematic phase errors that predominantly distort the imaginary part of the extracted conductivity spectrum. Numerical analysis reveals that temporal deviations of only several tens of femtoseconds can induce substantial errors in the retrieved conductivity, particularly for ultrathin films. Lorentz-model analysis further indicates that the timing-induced phase error primarily alters the high-frequency permittivity parameter, whereas the resonance frequency, oscillator strength, and damping constant remain comparatively stable. Artificial temporal-offset analysis further shows that physically reasonable dielectric responses are obtained only within a narrow temporal-alignment range near zero delay. These results demonstrate the critical importance of precise temporal alignment for accurate THz-TDS characterization of ultrathin materials and devices and highlight the need to account for timing-induced phase errors in conductivity extraction.</p>