require(survival)
D = rbinom(100, 1, 0.5)
Y = rbinom(100, 1, 0.5)
X = rexp(100, 1)
Delta = rbinom(100, 1, 0.5)
coxph(Surv(time = X, event = Delta) ~ D + offset(Y) + offset(D:Y))

> coxph(Surv(time = X, event = Delta) ~ D + offset(Y) + offset(I(D*Y)))
Call:
coxph(formula = Surv(time = X, event = Delta) ~ D + offset(Y) +
    offset(I(D * Y)))

     coef exp(coef) se(coef)      z        p
D -0.9959    0.3694   0.2825 -3.525 0.000423

Likelihood ratio test=12.26  on 1 df, p=0.0004635
n= 100, number of events= 53