That makes sense for durations, but a time_t is actually a timestamp. A proper time can have the same units as a duration, but is only useful in conjunction with information about the trajectory of the clock measuring the time. If space is continuous, the amount of information to specify the trajectory is limited only by the capabilities of the measurement apparatus. Of course, for all we know at this point spacetime is quantized; so we should probably hold off on standardizing the time format to end all time formats.