Fluorite | Level 6

## Calculating difference in dates using the same date variable

Hello,

I want to calculate the difference between dates in days using the same variable date [Procedure_Date] in the same patient, among only those with the same Status=Alive.

data have;

 patient_num Procedure_Date Status_Date Status patient33 03/07/2002 02/23/2004 Alive patient33 02/23/2004 10/01/2014 Alive patient50 05/07/2007 12/02/2014 Alive patient50 05/27/2009 10/08/2015 Dead

data want;

 patient_num Procedure_Date Status_Date Status Days_Between_Procedure patient33 03/07/2002 02/23/2004 Alive 718 patient33 02/23/2004 10/01/2014 Alive 718 patient50 05/07/2007 12/02/2014 Alive . patient50 05/27/2009 10/08/2015 Dead .

So for example, the difference in days of patient33's first Procedure_Date and second Procedure_Date, which equals to 718 days (same Status="Alive") and appears in the both rows.

Is there a way to do it in SAS datastep or in SQL?

Thank you!

3 REPLIES 3
Super User

## Re: Calculating difference in dates using the same date variable

It appears that you only want the first value calculated and then retained. If your data is sorted by patient_num and procedure_date and your procedure_date and status_date variables are SAS date valued variables

```data want;
set have;
by patient_num;
retain  Days_between_procedure;
if first.patient_num then Days_between_procedure= Status_date-Procedure_date;
run; ```

If your dates are character variables then you will have to convert them to SAS date values

`if first.patient_num then Days_between_procedure=  input(Status_date,mmddyy10.)- input(Procedure_date,mmddyy10.);`
PROC Star

## Re: Calculating difference in dates using the same date variable

Putting the same "days between procedures" on every observation is probably a mistake.  What values go where, if the patient actually had 3 procedures instead of 2?

Here's an easy way to put the "days between procedures" on the current observation only, as long as the patient is still alive.

proc sort data=have;

by patient_num procedure_date;

run;

data want;

set have;

by patient_num procedure_date;

days_between_procedure = dif(procedure_date);

if first.patient_num or status ne 'Alive' then days_between_procedure = .;

run;

You may want to inspect the results to see if they seem like they would be easy enough to work with.  There are other ways, if you really need them.

Tourmaline | Level 20

## Re: Calculating difference in dates using the same date variable

data have;
input patient_num : \$10. (Procedure_Date Status_Date) (:mmddyy10.) Status \$;
format Procedure_Date Status_Date mmddyy10.;
datalines;
patient33 03/07/2002 02/23/2004 Alive
patient33 02/23/2004 10/01/2014 Alive
patient50 05/07/2007 12/02/2014 Alive
;

data want;
call missing(_flag);
do _n_=1 by 1 until(last.patient_num);
set have;
by patient_num Procedure_Date;
if _n_=2 then _temp=Procedure_Date;
end;
do _n_=1 by 1 until(last.patient_num);
set have;
by patient_num Procedure_Date;
if _n_=1 and missing(_flag) then Days_Between_Procedure=intck('days',Procedure_Date, _temp);
output;
end;
drop _:;
run;

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