git could also have just followed some standards. For instance, how am I supposed to know the syntax of "3 days ago" in general?

They could have used time spans from ISO 8601 standard, so for instance simply "P3D". Or "P3Y6M4DT12H30M17S".

git is powerful but it involves a higher amount of time investment than it could if it were not reinventing the wheel and taking shortcuts as much. It's easy to see that it is and was built for powerusers mainly.