I'm having some trouble fitting a model for differences-in-differences regression in Stata. Specifically, I'm confused as to the B3 coefficient in the below:

y = B0 + B1p + B2trt + B3(p*trt) + u

Here, "p" and "trt" are dummy variables where p=0 for pre-treatment and 1 for post-treatment, and trt=0 for control group and 1 for treatment group. "p*trt" is an interaction term indicating the marginal impact of the treatment group over the experimental period. My question is whether this should be a simple dummy variable equal to zero if either p/trt=0 and 1 if both p/trt=1.

Any help is much appreciated.

Thanks!