how to assign previous date

[thanikondharish]

data s1 ;
input id $ start_date:date9. end_date:date9. ;
format start_date end_date date9. ;
cards ;
101 23sep2018  24sep2018
101 25sep2018  27sep2018
101 .                  30sep2018
101 02oct2018  04oct2018
101 .                  08sep2018
;

Q1)in above dataset start_date has some missing values then how to assign previous values of end_date variable?

id	start_date	end_date
101	23SEP2018	24SEP2018
101	25SEP2018	27SEP2018
101	27SEP2018	30SEP2018
101	02OCT2018	04OCT2018
101	04OCT2018	08SEP2018

Q2)in above dataset start_date has some missing values then how to assign previous values of end_date variable and add one day see below dataset?

id	start_date	end_date
101	23SEP2018	24SEP2018
101	25SEP2018	27SEP2018
101	28SEP2018	30SEP2018
101	02OCT2018	04OCT2018
101	05OCT2018	08SEP2018

[Kurt_Bremser]

SAS dates are counts of days, so simple additions and subtractions will do it.

[Astounding]

data want;

set have;

by id;

prior_end = lag(end_date);

if start_date = . and first.id = 0 then do;

   * For Q1;

   start_date = prior_end;

   * For Q2;

   start_date = prior_end + 1;

end;

drop prior_end;

run;