A narrow window stays open while the light still holds. Honoring the present as it stands now, before dusk comes and the mountains go black under stumbling feet, is the whole of what is asked. The warning points less to a coming punishment than to a moment that will not stay open forever.
Many people spend a lifetime looking for light somewhere ahead, in the next achievement or the next arrival, while the light already here quietly fades. The mind keeps postponing, certain of more time. Then the dark settles in, and the very thing that was searched for turns to shadow. What could have been a simple waking in daylight becomes a long groping through the gross dark.