I wrote this code , but it doesnt seem to work completely :   data want ; set have ; by PatientID ; StartDate2 = StartDate ; retain StartDate2 ; format new_enddate new_startdate StartDate2 mmddyy10. ; if (EndDate - StartDate) ge 180 then do ; new_startdate = StartDate2 ; new_enddate = sum(new_startdate,180); StartDate2 = StartDate +181 ; end; else if (EndDate - StartDate2) lt 180 then do; new_enddate = sum(StartDate2,180) ; new_startdate = StartDate2 ; end ; run ;   RAW DATA  Obs PatientID LineName StartDate EndDate1234567891011121314151617181920 F000678A98792 Gemcitabine,Paclitaxel Protein-Bound 02/01/2016 02/11/2016 F002F4C07392F FOLFIRINOX 08/19/2015 04/05/2016 F002F4C07392F Gemcitabine,Paclitaxel Protein-Bound 04/06/2016 05/11/2016 F00314B361B6A Gemcitabine 10/12/2015 11/27/2016 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 11/28/2016 08/21/2017 F00314B361B6A FOLFOX 08/22/2017 03/07/2018 F003E7ADEB2F8 Gemcitabine,Paclitaxel Protein-Bound 01/20/2017 02/03/2017 F006A77E4CDB0 FOLFIRINOX 07/12/2017 08/27/2017 F006A77E4CDB0 Pembrolizumab 08/28/2017 12/07/2017 F007499692F0D FOLFIRINOX 08/28/2017 10/31/2017 F007499692F0D Gemcitabine,Paclitaxel Protein-Bound 11/01/2017 03/26/2018 F0080CAE5E872 Gemcitabine,Paclitaxel Protein-Bound 02/12/2014 04/19/2017 F00C081DC6BC4 Gemcitabine,Paclitaxel Protein-Bound 02/17/2017 08/13/2017 F00C081DC6BC4 Capecitabine,Gemcitabine 08/14/2017 11/09/2017 F00CC20661EE3 Gemcitabine,Paclitaxel Protein-Bound 10/19/2015 05/03/2016 F00CC20661EE3 FOLFIRINOX 05/04/2016 05/09/2016 F00E0ED1D3143 FOLFIRINOX 09/21/2016 01/02/2017 F00E0ED1D3143 Fluorouracil,Gemcitabine,Irinotecan,Leucovorin,Oxaliplatin,Paclitaxel Protein-Bound 01/03/2017 03/27/2018 F00F92CD2DB8C FOLFOX 06/09/2016 06/23/2016 F01147636BCCA Gemcitabine,Paclitaxel Protein-Bound 08/04/2015 11/03/2015 EPISODE RULE     I want to see the values in StartDate2 to appear in new_startdate .. and tehn add 180 to them to get the new_enddate..    Obs PatientID LineName StartDate EndDate new_startdate new_enddate StartDate21234567891011121314151617181920 F000678A98792 Gemcitabine,Paclitaxel Protein-Bound 02/01/2016 02/11/2016 02/01/2016 07/30/2016 02/01/2016 F002F4C07392F FOLFIRINOX 08/19/2015 04/05/2016 02/16/2016 02/15/2016 08/19/2015 F002F4C07392F Gemcitabine,Paclitaxel Protein-Bound 04/06/2016 05/11/2016 04/06/2016 10/03/2016 04/06/2016 F00314B361B6A Gemcitabine 10/12/2015 11/27/2016 04/10/2016 04/09/2016 10/12/2015 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 11/28/2016 08/21/2017 05/28/2017 05/27/2017 11/28/2016 F00314B361B6A FOLFOX 08/22/2017 03/07/2018 02/19/2018 02/18/2018 08/22/2017 F003E7ADEB2F8 Gemcitabine,Paclitaxel Protein-Bound 01/20/2017 02/03/2017 01/20/2017 07/19/2017 01/20/2017 F006A77E4CDB0 FOLFIRINOX 07/12/2017 08/27/2017 07/12/2017 01/08/2018 07/12/2017 F006A77E4CDB0 Pembrolizumab 08/28/2017 12/07/2017 08/28/2017 02/24/2018 08/28/2017 F007499692F0D FOLFIRINOX 08/28/2017 10/31/2017 08/28/2017 02/24/2018 08/28/2017 F007499692F0D Gemcitabine,Paclitaxel Protein-Bound 11/01/2017 03/26/2018 11/01/2017 04/30/2018 11/01/2017 F0080CAE5E872 Gemcitabine,Paclitaxel Protein-Bound 02/12/2014 04/19/2017 08/12/2014 08/11/2014 02/12/2014 F00C081DC6BC4 Gemcitabine,Paclitaxel Protein-Bound 02/17/2017 08/13/2017 02/17/2017 08/16/2017 02/17/2017 F00C081DC6BC4 Capecitabine,Gemcitabine 08/14/2017 11/09/2017 08/14/2017 02/10/2018 08/14/2017 F00CC20661EE3 Gemcitabine,Paclitaxel Protein-Bound 10/19/2015 05/03/2016 04/17/2016 04/16/2016 10/19/2015 F00CC20661EE3 FOLFIRINOX 05/04/2016 05/09/2016 05/04/2016 10/31/2016 05/04/2016 F00E0ED1D3143 FOLFIRINOX 09/21/2016 01/02/2017 09/21/2016 03/20/2017 09/21/2016 F00E0ED1D3143 Fluorouracil,Gemcitabine,Irinotecan,Leucovorin,Oxaliplatin,Paclitaxel Protein-Bound 01/03/2017 03/27/2018 07/03/2017 07/02/2017 01/03/2017 F00F92CD2DB8C FOLFOX 06/09/2016 06/23/2016 06/09/2016 12/06/2016 06/09/2016 F01147636BCCA Gemcitabine,Paclitaxel Protein-Bound 08/04/2015 11/03/2015 08/04/2015 01/31/2016 08/04/2015 I want it to look like this : obs Patient ID LineName StartDate EndDate 1 F000678A98792 Gemcitabine,Paclitaxel Protein-Bound 2/1/2016 7/1/2016 2 F002F4C07392F FOLFIRINOX 8/19/2015 1/19/2016 3 F002F4C07392F FOLFIRINOX 1/20/2016 6/20/2016 4 F002F4C07392F Gemcitabine,Paclitaxel Protein-Bound 4/6/2016 5/11/2016 5 F00314B361B6A FOLFOX 8/22/2017 1/22/2017 6 F00314B361B6A FOLFOX 1/23/2017 3/7/2018 7 F00314B361B6A Gemcitabine 10/12/2015 3/12/2016 8 F00314B361B6A Gemcitabine 3/13/2016 8/13/2016 9 F00314B361B6A Gemcitabine 8/14/2016 11/27/2016 10 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 11/28/2016 4/28/2016 11 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 4/29/2016 9/29/2016 12 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 9/30/2016 3/2/2017 13 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 3/3/2016 8/3/2017 14 F00314B361B6A Gemcitabine,Paclitaxel Protein-Bound 8/4/2017 8/21/2017 ... View more

Obs Pat_id              Diag_date    LineName                                                StartDate EndDate  episode datebirrth year episode year age 1 FDD586EC54DFF 05/05/2016 Gemcitabine,Paclitaxel Protein-Bound 05/16/2016 05/26/2016 05/16/2016 1956 2016 60 2 FCF06DFFE96D3 06/09/2016 Gemcitabine,Paclitaxel Protein-Bound 07/22/2016 08/22/2016 07/22/2016 1942 2016 74 3 F468936773849 10/15/2015 FOLFIRI 01/13/2017 06/05/2017 10/19/2015 1955 2015 60 4 F468936773849 10/15/2015 Gemcitabine,Paclitaxel Protein-Bound 10/19/2015 01/12/2017 10/19/2015 1955 2015 60 5 F91E7F27902F2 07/23/2015 FOLFOX 04/11/2017 07/22/2017 08/17/2015 1958 2015 57 6 F91E7F27902F2 07/23/2015 Fluorouracil,Irinotecan Liposomal,Leucovorin 09/27/2016 04/10/2017 08/17/2015 1958 2015 57 7 F91E7F27902F2 07/23/2015 Gemcitabine,Paclitaxel Protein-Bound 08/17/2015 09/26/2016 08/17/2015 1958 2015 57 8 FC72D24D879B2 09/09/2016 FOLFOX 01/11/2017 05/30/2017 09/27/2016 1933 2016 83 9 FC72D24D879B2 09/09/2016 Fluorouracil,Irinotecan Liposomal,Leucovorin 05/31/2017 01/02/2018 09/27/2016 1933 2016 83 10 FC72D24D879B2 09/09/2016 Gemcitabine,Paclitaxel Protein-Bound 09/27/2016 01/10/2017 09/27/2016 1933 2016 83 11 FA6DB21197963 01/04/2016 FOLFIRINOX 01/29/2016 03/24/2018 01/29/2016 1951 2016 65 12 F985921E439A3 05/08/2014 FOLFOX 05/28/2014 08/31/2014 05/28/2014 1935 2014 79 13 FC2B6C5BFDA37 08/06/2015 FOLFIRINOX 09/10/2015 02/02/2016 09/10/2015 1943 2015 72 14 FAE07E0051C78 11/17/2015 FOLFIRINOX 12/08/2015 04/20/2017 12/08/2015 1945 2015 70   This is what my resullt looks like but i want to see that for id F468936773849  the start date for FOLFIRI is 01/13/2017 and for Gemcitabine,Paclitaxel Protein-Bound it is 10/19/2015.  it shouold be differnt for differnet treatments and also follow the episode rule of 6 months, so for example for id:- F91E7F27902F2 the patinet is undergoing 3  differnet treatments , I want to see theStartdate for each treatment it, for FOLFOX the epideosde start date should be  04/11/2017 adn not  08/17/2015. SAS is taking the earliest startdate for the patient accoridng to his first treatment.   ... View more

this is somewhat what the code looks like : if first.enrolid or dtstart >sum (lag(dtend).,1) then dtstat_beg= dtstart retain dtstart_beg ; if last.enrolid or dtstart_next>dtend+1 ; ... View more