Time series models


No cake for spunky
I have a lot of analysis where X is measured at one point, sometimes years before Y is measured. I remain unclear when a time series model is required or not in this case assuming you are not using lags of Y to predict. Is it when X and/or Y is not stationary?

That is my guess but I am not sure this is the way analysis is commonly done, because I am not certain stationarity test are even done commonly.