First and stop date - hours in difference

[monday89]

Hello. I have the following dataset per subject ID where i need to calculate difference in hours based on each day. Logic:

• For day = 0, where i need to take the MAX DRUG_stop_DTM - Enrolled_DTM.
• Next days (EXCEPT THE LAST DAY for that subjectID), the difference in hours needs to be static set to 24 hours. 
• Last day: final_stop_DTM - first drug_start_dtm for that last day. If the "Final_stop_dtm" is missing then use max drug_stop_dtm for that day 

Have:

PatientID	Enrolled_DTM	days	Drug_start_DTM	Drug_stop_STM	WEIGHT	final_stop_DTM
1232	26MAR2020:13:30:00	0	26MAR20:13:30:00	26MAR20:23:59:59	74.2	28MAR2020:21:56:00
1232	26MAR2020:13:30:00	1	27MAR20:00:00:00	27MAR20:23:59:59	74.2	28MAR2020:21:56:00
1232	26MAR2020:13:30:00	1	27MAR20:03:46:00		74.2	28MAR2020:21:56:00
1232	26MAR2020:13:30:00	1	27MAR20:20:04:00		74.2	28MAR2020:21:56:00
1232	26MAR2020:13:30:00	2	28MAR20:00:00:00	28MAR20:13:00:00	74.2	28MAR2020:21:56:00
1232	26MAR2020:13:30:00	2	28MAR20:04:11:00		74.2	28MAR2020:21:56:00
1232	26MAR2020:13:30:00	2	28MAR20:04:46:00		74.2	28MAR2020:21:56:00
4456	02JAN2020:16:11:00	0	02JAN20:18:16:00	02JAN20:23:59:59	80.9	.
4456	02JAN2020:16:11:00	1	03JAN20:00:00:00	03JAN20:15:50:00	80.9	.
4456	02JAN2020:16:11:00	1	03JAN20:15:50:00	03JAN20:23:59:59	80.9	.
4456	02JAN2020:16:11:00	2	04JAN20:00:00:00	04JAN20:21:11:00	80.9	.

want;

PatientID	days	WEIGHT	hours_difference
1232	0	74.2	10.5
1232	1	74.2	24
1232	2	74.2	22
4456	0	80.9	7.82
4456	1	80.9	24
4456	2	80.9	21.18

What I have done so far is break the dataset by days. for day = 0 but then i wasn't sure how to do the "last" by group per patient

any help is appreciated. 

[LeonidBatkhan]

Hi monday89,

Here is how you can set up you logic:

proc sort data=A out=B; 
   by PatientID Days;
run;

data C;
   set B;
   by PatientID Days;
   if first.Days then
   do;
      /* do your first observation (Days=0) processing */
   end;
   else
   if last.Days then
   do;
      /* do your last day processing */
   end;
   else
   do;
      /* do your processing for other days except last */
   end;
run;

Hope this helps.