Programming the statistical procedures from SAS

Create a time-interaction covariate in counting process to test proportional hazard assumption

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Create a time-interaction covariate in counting process to test proportional hazard assumption

I want to create time-interaction covariate to test whether covariates in model meet proportional hazard assumption.
First few rows of input dataset is as follows:
T1 & T2 correspond to entry and exit time of counting process style of input.
Status is time-dependent variable while COV1-COV3 are time-fixed covariates (continuous or categorical).
Event (0 or 1) and Time indicates censoring status and censored time, respectively.

 

IDT1T2StatusEventTimeCOV1COV2COV3
1010A0413114F
11030B0413114F
13041C1413114F
20107B01512518M
2107151C01512518M

PHREG model in SAS looks like below:

 

proc phreg data=have; 
class status(ref='A') COV3(ref='F');
model (T1,T2)*Event(0)=Status COV1 COV2 COV3 COV1_t; 
COV1_t = ; run;

At this point, how can I express time-interaction covariate in programming statement of PHREG procedure?
a) COV1_t = (T2-T1)*COV1
b) COV1_t = T2*COV1
c) COV1_t = Time*COV1
I think c) is right way when compared with usual phreg model, but b) seems plausible as well.

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