**is it possible to do a time series analysis with more than 1 explanatory variable?**

I know in the simplest (using the word simple very loosely) terms time series involves modeling Y = time, where time is essentially the only information used to try and predict Y (based on trends, seasonality....).

However, what if I am interested in using time AND population to predict cost?

If I were trying to do some type of normal regression I would have:

model cost = time population

Is there some way to do something like this in time series? I am ultimately curious about forecasting costs based not only future months but future population projections. I have not come across anything in SAS or JMP that seems to account for more variables other than time.