When God set this planet in motion in Gen 1 revolving on its axis to establish days and nights (hours), and around the sun on a tight schedule to establish years, and the moon revolving around the earth to establish months, and other signs, man had a means to comprehend time, and measure "Day and Night". Without physical objects to relate to all there could be is eternity, timeless existence without beginning or end, since eternity doesn't depend upon matter. Think of eternity as perpetual "Now". We know that because the Word was before the creation, while we are here but a brief moment relatively.
How could early man measure time? As a Boy Scout I remember one of the first things we did in a camp was to put a stick in the ground to make a shadow, and peg the shadow so we could see how far it moved along the arc until sunset. From that we could construct a sun dial divided into hours, putting a stone at the tip of the noon shadow, the closest part of the arc to the middle stick.. The longest shadows marked morning and evening. Amazingly, the pattern repeated every day while camping.