calculate the difference between two instances in days by id

i am trying to create code that recognizes pattern. Specifically if a person makes certain buying behavior every year at the same month (ex : Christmas shopping done on black Friday), and I want to eliminate those shoppers from my analysis.

the variables I have(sample) are :

Dates: Person_id : shopping_item: 20150301 1 10 20140301 1 10

And I am not sure how to code it in Matlab. So far I am thinking the following way: 1) Unique dates and id’s dates = unique(Z.date); ndates = length(dates); insider_id = unique(PersonID); insider_id(isnan(insider_id)) = []; ninsids = length(insider_id);

2) index flt_1 = dates > 0 & insider_id>0;

3) loop (That’s where I need MOST help with THE LOGIC)

for j=1:length(Z.flt_1)

if Z.flt_0(j)>0 

diff(Z.date (j), Z.insider_id(j)) = diff(date(j), Z.insider_id(j)) - diff(date(j)-1, Z.insider_id(j))

z= diff=>335 & diff<=395;

pattern=diff.*z; 

end

end