I've had a quick look at the 744 manuals and wiring diagrams, it's not 100% clear but ACARS gets it's time input from the CMC, the wiring diagram shows that the CMC gets an input from the flt deck clocks. As far as I know the clocks are set manually, but this is not accurate as it is a manual input. I would hazard a guess that there is some input from SATCOM which will be more accurate, I just can't find the input at the moment.
