SAS do loop with if statement

I am trying to group by dataset in three month groups, or quarters, but as I'm starting from an arbitrary date, I cannot use the quarter function in sas.

Example data below of what I have and quarter is the column I need to create in SAS. The start date is always the same, so my initial quarter will be 3rd Sep 2018 - 3rd Dec 2018 and any active date falling in that quarter will be 1, then quarter 2 will be 3rd Dec 2018 - 3rd Mar 2019 and so on. This cannot be coded manually as the start date will change depending on the data, and the number of quarters could be up to 20+.

The code I have attempted so far is below

data test_Data_op;
set test_data end=eof;
%let j = 0;
%let start_date = start_Date;
if &start_Date. <= effective_dt < (&start_date. + 90) then quarter = &j.+1;
run;

This works and gives the first quarter correctly, but I can't figure out how to loop this for every following quarter? Any help will be greatly appreciated!