Essentially, I have data from a PK sampling study that I would like to use to calculate time after last dose administration:

For example, it looks like this:
ID DATE TIME CONC DOSE
290261 02/14/2002 20:24 . 4
290261 02/15/2002 9:21 278.1000061 0
290261 02/15/2002 11:02 2202.399902 0
290261 02/15/2002 12:00 1938.5 0
290261 02/15/2002 13:55 1476.300049 0
290261 02/15/2002 17:45 310.2000122 0
290261 02/15/2002 21:50 120.1999969 0
290261 02/16/2002 10:04 37.70000076 0
290261 03/02/2002 20:38 . 4
290261 03/04/2002 15:36 62.59999847 0
290261 04/21/2002 20:38 . 4
290261 04/23/2002 16:19 77.80000305 0
290261 05/19/2002 21:21 . 4
290261 05/21/2002 17:02 64.90000153 0
290261 06/16/2002 21:07 . 4
290261 06/18/2002 16:04 65.69999695 0
290261 08/11/2002 21:36 . 4
290261 08/13/2002 15:07 79.5 0
290261 10/06/2002 20:24 . 4
290261 10/08/2002 15:36 72.69999695 0
290261 01/05/2003 20:24 . 4
290261 01/07/2003 17:02 60.70000076 0
381127 05/29/2002 19:55 . 4
381127 05/30/2002 8:38 198.1000061 0
381127 05/30/2002 12:28 1911.300049 0
381127 05/30/2002 13:26 1704.300049 0
381127 05/30/2002 15:21 790.4000244 0
381127 05/30/2002 19:26 203.1000061 0
381127 05/30/2002 23:31 85.30000305 0
381127 05/31/2002 11:31 22.39999962 0
381127 07/08/2002 21:07 . 4
381127 07/10/2002 15:07 98.90000153 0
381127 08/09/2002 19:55 . 4
381127 08/10/2002 10:19 130.6999969 0
381127 08/10/2002 11:02 137.1000061 0
381127 08/10/2002 12:00 2444 0
381127 08/10/2002 13:55 2044.900024 0
381127 08/10/2002 15:07 1325 0
381127 08/10/2002 18:57 676.2999878 0
381127 08/10/2002 23:02 391.8999939 0
381127 10/02/2002 19:55 . 4
381127 10/04/2002 15:36 96.19999695 0
400873 12/12/2001 0:00 . 4
400873 12/13/2001 7:55 307.5 0
400873 12/13/2001 9:07 2287.300049 0
400873 12/13/2001 10:04 1872 0
400873 12/13/2001 12:14 1086.699951 0
400873 12/13/2001 16:04 378.2999878 0
400873 12/13/2001 20:24 70.30000305 0
400873 12/14/2001 8:09 169.6000061 0
400873 01/08/2002 0:00 . 4
400873 01/11/2002 12:43 206.1999969 0
400873 01/22/2002 21:07 . 4
400873 01/23/2002 8:09 324.7999878 0
400873 01/23/2002 9:50 966 0
400873 01/23/2002 10:33 1098.699951 0
400873 01/23/2002 12:43 2227.600098 0
400873 01/23/2002 16:33 170.1999969 0
400873 01/23/2002 20:24 61.20000076 0
400873 01/24/2002 8:24 285.3999939 0
400873 02/18/2002 21:07 . 4
400873 02/20/2002 14:38 113.3000031 0
400873 03/23/2002 21:07 . 4
400873 03/25/2002 12:28 112.5 0
400873 04/20/2002 21:07 . 4
400873 04/22/2002 12:57 123.1999969 0
400873 06/08/2002 21:07 . 4
400873 06/10/2002 9:50 265.2999878 0
400873 08/03/2002 21:07 . 4
400873 08/05/2002 12:00 179 0
400873 10/26/2002 21:07 . 4
400873 10/28/2002 10:19 161.5 0
400880 02/19/2002 6:28 . 4
400880 02/20/2002 7:26 428.6000061 0
400880 02/20/2002 8:24 3770.100098 0
400880 02/20/2002 9:36 2738.100098 0
400880 02/20/2002 11:31 2614.699951 0
400880 02/20/2002 15:50 1905.800049 0
400880 02/20/2002 19:26 1314 0
400880 02/21/2002 7:40 322.1000061 0
400880 03/03/2002 12:00 . 4
400880 03/04/2002 11:45 1579.5 0
400880 03/24/2002 9:07 . 4
400880 03/25/2002 11:16 273.5 0
400880 04/30/2002 6:43 . 4
400880 05/01/2002 6:43 417.6000061 0
400880 05/01/2002 8:24 3212.100098 0
400880 05/01/2002 9:21 3422.5 0
400880 05/01/2002 11:31 2620.100098 0
400880 05/01/2002 15:21 1771.400024 0
400880 05/01/2002 19:26 1031.5 0
400880 05/02/2002 7:26 297.7999878 0
400880 06/19/2002 21:07 . 4
400880 06/21/2002 9:50 1383.5 0
400880 08/17/2002 22:04 . 4
400880 08/19/2002 11:02 1457.099976 0
400880 10/05/2002 23:02 . 4
400880 10/07/2002 14:52 262.7000122 0
400880 01/04/2003 22:33 . 4
400880 01/06/2003 9:50 1375.099976 0
400881 04/09/2002 12:57 . 4
400881 04/10/2002 7:12 86.19999695 0
400881 04/10/2002 8:24 2325.800049 0
400881 04/10/2002 9:21 1963.5 0
400881 04/10/2002 11:16 836.4000244 0
400881 04/10/2002 15:21 215.5 0
400881 04/10/2002 19:26 97.80000305 0
400881 04/11/2002 7:26 37.70000076 0
400881 04/28/2002 21:07 . 4
400881 04/29/2002 10:19 1015.799988 0
400881 06/01/2002 22:04 . 4
400881 06/03/2002 11:31 287.2000122 0
400881 06/25/2002 7:55 . 4
400881 06/26/2002 6:57 53.70000076 0
400881 06/26/2002 8:09 1650.800049 0
400881 06/26/2002 9:07 2553.699951 0
400881 06/26/2002 11:02 913.7999878 0
400881 06/26/2002 15:07 251.8000031 0
400881 06/26/2002 19:12 135.6000061 0
400881 06/27/2002 7:12 54 0
400881 10/05/2002 21:07 . 4
400881 10/07/2002 14:38 113.5999985 0
400881 11/30/2002 21:07 . 4
400881 12/02/2002 11:45 0 0
400881 03/01/2003 21:07 . 4
400881 03/03/2003 10:04 0 0
400882 02/06/2002 21:07 . 4
400882 02/07/2002 7:26 163.1000061 0
400882 02/07/2002 8:38 969.0999756 0
400882 02/07/2002 9:36 1946 0
400882 02/07/2002 11:31 1011.900024 0
400882 02/07/2002 15:36 298.5 0
400882 02/07/2002 19:40 110.0999985 0
400882 02/08/2002 7:40 42.20000076 0
400882 02/23/2002 21:07 . 4
400882 02/25/2002 11:31 81.30000305 0
400882 03/23/2002 21:07 . 4
400882 03/25/2002 11:31 195.1999969 0
400882 04/30/2002 21:07 . 4
...

where DOSE = 4 means it was a dosing event while DOSE = 0 means observation event. I would like to make a simple "Time after Dose" vs "Concentration" plot for each individual in the study.

Any input would be appreciated.