Let $X$ be a one-dimensional diffusion and let $g\colon[0,T]\times\mathbb{R}\to\mathbb{R}$ be a payoff function depending on time and the value of $X$. The paper analyzes the inverse optimal stopping problem of finding a time-dependent function $π:[0,T]\to\mathbb{R}$ such that a given stopping time $τ^{\star}$ is a solution of the stopping problem $\sup_τ\mathbb{E}\left[g(τ,X_τ)+π(τ)\right]\,.$ Under regularity and monotonicity conditions, there exists a solution $π$ if and only if $τ^{\star}$ is the first time when $X$ exceeds a time-dependent barrier $b$, i.e. $τ^{\star}=\inf\left\{ t\ge0\,|\,X_{t}\ge b(t)\right\} \,.$ We prove uniqueness of the solution $π$ and derive a closed form representation. The representation is based on an auxiliary process which is a version of the original diffusion $X$ reflected at $b$ towards the continuation region. The results lead to a new integral equation characterizing the stopping boundary $b$ of the stopping problem $\sup_τ\mathbb{E}\left[g(τ,X_τ)\right]$.