放不放在class语句里就看这个变量是不是分类变量。如果属于分类变量就要放进class语句。

楼上说得没错，如果模型里的time是continuous variable，那么class语句中不应加time；相反，如果模型里的time是discrete variable，那么class语句中要加time。在time只取0/1的情况下，无论是哪种类型的变量，估计出的结果是相同的。
In particular, if time is treated as a discrete variable, you should remove time*time and time*time*group from the model specification presented above, since these two variables will be collinear with time and time*group, respectively.

更正一下，用quantity variable versus categorical variable 来区分比较好，请忽略上面 continuous variable vs. discrete variable 的划分。

I think continuous is just contrast to discrete.
In this case, clearly time was treated as continuous variable and thus should not be included in CLASS statement.
JingJu