... first thing that came to mind was that quote.
We think we have the whole time to do what we want, we procrastinate the important things that needs to be done forgetting that time is not on our side.
The sunset is like a reminder for us to "Make hay while the sun shines" so as not to regret not doing what you should have when needed. The sunset signifies the end of another day and a time for us to think back to how the day went and what we were able to achieve for that day.