coxph(Surv(time1, time2, outcome)
                ~ X1 + X2 + X3 + X4 +
                  tt(X2) + tt(X2) + tt(X3) + tt(X3) +
                  strata(sites),
                  x = TRUE, y = TRUE, data = data,
                  tt = function(x,t,...){x*t})