I am going to assume $N$ is large and $x$ is small $(<1)$. Let $f:\\{0,\dots,N\\}\to\mathbb{Z}$ be the function $f(n)=\lfloor t+ n x\rfloor$. Let $n_1\geq 1$ be the first $n$ such that $f(n)>f(n-1)$ and $n_2$ be the last such $n$. Then $x\approx \frac{f(n_2)-f(n_1)}{n_2-n_1}$ and $t\approx f(0)+1-n_1x$.