I solved this by using
OS.get_unix_time() as the "zero point" for the start of my game time. When advancing the game time I add incrementally as much time (in seconds) as I want the game universe to move forward. Using the start time for comparison, I calculate how many seconds, minutes, hours, or days have passed.
Unfortunately there is no reverse function to determine the datetime from a UNIX timestamp, but at least it gives you an easier way to do the math.