In paper calendars, every day still has a small icon in the right or left-hand corner. It represents the phase of the moon for each day, and this waxing and waning accompanies the passing of time, in a continuous cycle that restarts roughly every 29 days.
The Moon Time timepiece app you see above shows you, at each visit, how many days until the next full moon and how many remain until the new moon reappears.
The number inside the circle shows the number of days elapsed since the last new moon.