The average of five days is the middle number of days.
That is 55/5= 1 1.
It has been five days, that is, 91011213. Today it is 14.
If it is an odd number of days, you can directly divide the total number by the number of days, and the average value is the middle day.
If it is an even number, dividing by half the number of days is the sum of the first and last dates. I don't know how to explain others.