For the Galileo system it is required that a space clock time prediction be performed, covering the time interval (T_p) between two uploads. The time prediction accuracy of the space clock is therefore an important issue. The predictability of the Space Passive Hydrogen Maser (S-PHM) time error is evaluated by the RMS of the predicted time errors at the prediction time T_p: ΔT_RMS(T_p). A linear prediction model is used, corresponding to the absence of a frequency drift. The results show that: (1) the RMS time error can be evaluated from a priori knowledge of the clock's Allan deviation; (2) conversely, it is possible to extract the Allan deviation from the measurement of ΔT_RMS versus T_p; (3) the modelling of ΔT_RMS(T_p) of S-PHM based on the white frequency and flicker frequency noises appears to be particularly accurate: the difference between the model fit and the measured prediction accuracy is ≤10 ps RMS for T_m = 24 h; (4) for T_p = 4 h, the performance of the S-PHM is a factor of 4.5 better than the performance of the space rubidium frequency standard (S-RAFS). This has a dramatic effect on the probability of the Signal In Space Accuracy: assuming a Gaussian distribution the probability of a predicted time error ΔT≤1.5 ns for T_p = 4 h is 89% for the S-RAFS, while the same time error constitutes an absolute upper bound for the Galileo S-PHM.