Repetitive division problem

Hi ChemistB,

When proceeding with this dataset, another problem occurred. I was able with your help to link the change in port calls to a particular country and date. So the following dataset is created, see attached.

Now, I wanted to insert a column which includes a lagged factor (a delay) of the difference of 15 days. So that means that the first of January now has to be linked to the difference of the 15th of January. This is easy done, when you have a list representing only one country. If you just equalise the lagged value on the first of January to the 15th of January, it works for the beginning of the period covered but not at the end since it will just start considering the values of the next country in the list.

So the question is, how to insert a lagged factor without taking the values of the next country into account?

The attached file represents the countries Aruba and Belgium and are only 2 of the 132 considered countries.

Thank you in advance.
DanVG