The finite difference time domain method is a finite difference method for solving maxwell equations numerically. There are several pieces to it, but this is the root of my question
$H_{i +1/2 , j+1/2}^{n +1/2} = H_{i +1/2 , j+1/2}^{n -1/2} + \space ....$
when $n=0$ we see that the stencil go out of the the time domain ($t \geq 0$, but the point would be $t-\frac{\Delta t}{2}$), which in english means that we need information before the initial condition was specified to move forward in time. Some help would greatly be appreciated.