How can I define a parameter in relation to some elements in the variable

Example:

For all t, H(t) = Z(t+1) + Z(t).

Z(t) is the variable. And H(t) is the parameter calculated with the adjacent elements in the variable. How can I do this?

BTW, in this instance, what’s the ‘index domain’ of the parameter H, as the index t=1,2,……,24, i.e. the maximum element in Z is Z(24), if I define the ‘index domain’ of H as t directly, which means that there exists H(24) = Z(25)+Z(24), which should not be existed. Would this cause error or how can I limit the index domain of H to t = 1,2,…..,23?