Time is an odd thing for Brian while he's lost in the wilderness. He doesn't have anything like a clock or a watch that would allow him to keep an accurate track of time during the day. Of course, he can generally track morning, noon, afternoon, and night by the movement of celestial objects like the sun and moon. Though he doesn't have a calendar, Brian can keep track of days based on a simple day and night cycle, and he notes each passing day by making a mark on the stone that is near the door to his shelter. Unfortunately, the days blur together so much that the marks on the rock are essentially useless. It's something for him to do, but these marks hold little meaning to Brian.
As time passes, Brian adopts an alternate way of noting the passage of time. This alternate method sees Brian orienting himself around major events, which prove more meaningful to him than the tallies on the wall. Certain events are so traumatic or so crucial to Brian's survival that they are burned into his memory, and he keeps a mental journal of those events. Events like First Meat day and First Arrow day are examples of the way Brian now orients himself around these important occasions. While this method isn't an exact way of measuring the passage of time, it is better suited for Brian's new life, serving as a way for Brian to measure his maturation and growing set of skills. Individual days are typically void of variation, so they don't mean anything to him; however, learning some new survival tactic or skill is hugely important to Brian, so those are the days and times that are most meaningful to him.