Adding Incremental date from starting date

[Jordani]

Hi, 

I need to add the date incrementally from a certain starting date.  My data look like this

data try;
input Product factory Week quantity ;
datalines;A 1 1 3
A 1 1 4
A 1 1 5
A 1 1 6
A 1 1 3
A 1 1 5
A 1 1 5
A 1 2 6
A 1 2 5
A 1 2 5
A 1 2 4
A 1 2 5
A 1 2 4
A 1 2 5
B 1 1 3
B 1 1 2
B 1 1 3
B 1 1 2
B 1 1 3
B 1 1 2
B 1 1 3
B 1 2 5
B 1 2 4
B 1 2 5
B 1 2 5
B 1 2 4
B 1 2 5
B 1 2 5
;
run;

The week column tells the the number of the week. Each week is repeated for 7 rows to give information for 7 days. I want to the add the date incrementally  from a certain starting date. So the days change based on the week sequence and the circle need to be done for each product . 

The table I want should look like this 

product   factory       Week      Quantity      Date 

A       1        1       3        20200120
A       1        1       5        20200121
A       1        1       6        20200122

.

,

,
B       1        1       3        20200120
B       1        1       2        20200121
B       1        1       3        20200121


[Patrick · Accepted solution]

If I understand right then below should do (for a week starting on Monday).

data have;
  input Product $ factory Week quantity;
  datalines;
A 1 1 3
A 1 1 4
A 1 1 5
A 1 1 6
A 1 1 3
A 1 1 5
A 1 1 5
A 1 2 6
A 1 2 5
A 1 2 5
A 1 2 4
A 1 2 5
A 1 2 4
A 1 2 5
B 1 1 3
B 1 1 2
B 1 1 3
B 1 1 2
B 1 1 3
B 1 1 2
B 1 1 3
B 1 2 5
B 1 2 4
B 1 2 5
B 1 2 5
B 1 2 4
B 1 2 5
B 1 2 5
;

%let start_dt='20jan2020'd;
data want;
  set have;
  by product factory week;
  format date yymmddn.;
  if first.week then
    date=intnx('week.2',&start_dt,week-1,'b');
  else date+1;
run;

proc print;
run;