This thesis investigates high-precision GNSS time transfer using undifferenced and uncombined (UDUC) observation models and explores its extension to space applications. The work focuses on rigorous multi-frequency and multi-constellation modeling, integer ambiguity resolution, precise satellite clock and orbit treatment, and the exploitation of carrier-phase observations for stable time and frequency transfer. By avoiding observation differencing and linear combination, the UDUC framework preserves the original information content of GNSS measurements and supports flexible modeling of receiver clocks, satellite clocks, atmospheric delays, and hardware biases. The study demonstrates the potential of UDUC GNSS time transfer for terrestrial timing links as well as satellite-based timing and space geodesy applications.