The old calendar remembers appointments that the people involved have completely forgotten.

Written records keep what people do not. A date, a name, an appointment, a note in a margin: all of it stays exactly as it was while the memory dissolves entirely, so a page can end up knowing more about a period of your life than you do. That is unsettling and occasionally very useful. What you put in writing is what stands a chance of surviving.

Look at any old diary or calendar you still own before it gets thrown away. Write down two things from this month that a version of you in ten years would want to have.