शुक्लकृष्णे गती ह्येते जगतः शाश्वते मते।एकया यात्यनावृत्तिमन्ययाऽऽवर्तते पुनः॥
śukla-kṛṣṇe gatī hyete jagataḥ śāśvate mateekayā yātyanāvṛttim anyayāvartate punaḥ
Word by word
The verse says
These two, the bright and the dark, are held to be the world's lasting paths. By the one a person goes without returning; by the other, he comes back again.
In plain words
The two paths are called śāśvate — lasting, permanent features of how things work.
Not a rule made for a particular age or people. Not something anybody chose.
Two ways out, always there, and the chapter is about to say what to do with that information.
Where you've felt this · in study
You cram for an exam the night before and it works — you pass, respectably. Three weeks later you could not answer a single question from that paper. The chapter you actually understood back in October is still with you in March, and you have not revised it once.
Going deeper
Two permanent routes, and neither one is a reward or a punishment, which is the detail most easily missed.
Nothing in these verses describes a judge. The bright and the dark are presented as features of the terrain, the way a river has a current. What decides which one a person takes is not a verdict passed on them but the state they are actually in, which was built long before the moment of departure.
That fits everything the chapter said at the start about the final thought being the product of a whole life.
So the useful reading is not about calendars at all. Some things you acquire are built to stay and some are not, and the difference is usually visible while you are acquiring them, if you are willing to look.