Utility function of habit formation model In the habit formation RBC model, the utility can be given as follows:

$U(c_{t},n_{t}) = \ln(c_{t}) - \frac{\theta}{1+\upsilon} \bigg(\frac{n_{t}}{n_{t-1}^{\phi}}\bigg)^{1+\upsilon}$

When I do FOC wrt $n_t$, I'm confused about how to treat the variable $n_{t-1}$. Is it exogenous or should I try to find $\frac{\partial n_{t-1}}{\partial n_t}$?

Many thanks.
I believe $n_{t-1}$ is a quantity from the past, so you should be able to treat it as an exogenous variable