Before he left for his friend’s the man wound up his clock and set it at a particular time. He then noted the time of his arrival at his friend’s and the time of his departure. When he got home he noted the time of his arrival. Throughout the journey he made sure that he walked at the same pace. The time that had elapsed on his own clock represented the time it took to complete the two journeys there and back, plus the length of time that he stayed with his friend. As he knew the time that he had spent with his friend, he subtracted this from the time that had elapsed for the whole journey and then divided this in half. This gave him the time for his return journey from his friend’s house to his own. He then added this to the exact time he had left his friend’s house, so that he could set his own clock at the right time.
To illustrate this, let’s say he set his own clock at 10 am before he left his house. When he arrived at his friend’s house his friend’s clock was showing 3 pm and when he left it was showing 5 pm. Then, when he got home his clock was showing 2 pm. So the entire journey had taken four hours. As he had spent two hours with his friend, this meant that his journey back had taken an hour. Therefore, the correct time was 6 pm – the time that was showing on his friend’s clock before he left, 5 pm, plus the hour for the journey back.