Getting error message on SAS

[Ranjeeta]

data stroke_onset;
set EVT.stroke_ischemic;
by HCN REG_TM;
where REG_TM <= ADM_TM;
if last.HCN;
Window1=(REG_TM-SP340_ONSET)/60;
length cat $100.;
if 0<=Window1<360 then cat="0to6";
else if 360<=Window1<2880 then cat="6to48";
else if 2880<=Window1<7200 then cat = ">=48";
else if missing (SP340_ONSET) then cat = "onset time missing";
else if SP340_ONSET>= REG_TM then cat = "Stroke onset equal to and after Reg time";
else cat='NA';
run;

I get the following error message 

Please advise

ERROR: BY variables are not properly sorted on data set EVT.STROKE_ISCHEMIC.
HCN=0132045842 CIHI_KEY_NACRS=539852017100006511 CIHI_KEY_DAD=5144420171005013
REG_TM=16JAN2018:16:49:00 ADM_TM=16JAN2018:17:44:00 DISCH_TM=19JAN2018:09:45:00
FIRST_ADM_TM=16JAN2018:16:49:00 FIRST_ADM_DT=16JAN2018 FIRST_INST=3985 FIRST_FAC=852
FIRST_FAC_NAME=St Michael's Hospital SP340_FIELD=YNN01160000N201801161430 SP340_TIME=201801161430
SP340_MON=JAN SP340_ONSET=16JAN2018:14:30:00 SP340_FLAG=1 SP340_VALID=1 DEATH_DT=.
STROKE_MORTALITY30=0 FIRST.HCN=1 LAST.HCN=1 FIRST.REG_TM=1 LAST.REG_TM=1 Window1=. cat= _ERROR_=1
_N_=68
NOTE: Missing values were generated as a result of performing an operation on missing values.
Each place is given by: (Number of times) at (Line):(Column).
40 at 2426:16
NOTE: The SAS System stopped processing this step because of errors.
NOTE: There were 69 observations read from the data set EVT.STROKE_ISCHEMIC.
WHERE REG_TM<=ADM_TM;
WARNING: The data set WORK.STROKE_ONSET may be incomplete. When this step was stopped there were 62
observations and 21 variables.
WARNING: Data set WORK.STROKE_ONSET was not replaced because this step was stopped.
NOTE: DATA statement used (Total process time):
real time 0.85 seconds
cpu time 0.01 seconds

[Reeza]

You need to sort your data. Add a PROC SORT before this step and your BY statements in the two PROCS should match.

[singhsahab]

Hello,

You have to sort your data first 

or

use Notsorted option with By statement. 

   by HCN REG_TM notsorted;