From QED:
The factor by which time is stretched out (to make a 45° angle represent a particle going the speed of light) is called c, and you'll find c's flying around everywhere in Einstein's formulas—they are the result of the unfortunate choice of the second as the unit of time, rather than the time it takes light to go one meter.
This would make these formulas convenient, but I also wonder what it would be like to have a different general standard unit of time, like the light-meter.
Though the light-meter is 0.000000003335641 seconds, so maybe the light-terametre would be more commonly used. If you were going to meet someone in around an hour, you could say that you’d see them in 1.2 light-terametres. But maybe people would plan on meeting in one light-terametre because it’s convenient to say.