Shouldn't time-based (weekly and holiday) and trend effects be multiplied rather than added? For instance, if users spend twice as many hours on the weekend on

Shouldn't time-based (weekly and holiday) and trend effects be multiplied rather than added? For instance, if users spend twice as many hours on the weekend on a website, and the total number of users has doubled, then these effects multiply to give 4x visits than the baseline non-weekend.