Time-dependent changes of photo-statistic gains normalized to the nominal absolute gains used in the baseline MC model for the current detector model. This detector model was constructed in 2012, and reproduces the characteristics of VERITAS after the PMT camera upgrade. Black points show the average gain factors for each instrument epoch. Gray points show the individual unfiltered gain factor values per telescope and daily flasher run. Blue, orange, green and red points show the result of a median filter of the individual gain factors. Curves of the same colors represent a spline interpolation of the filtered values. Even though spline interpolation can locally diverge when there is no data available, we note that this occurs only during Summer breaks, where no data is taken in VERITAS, therefore it is not a concern for this study. Error bars represent statistical uncertainties.