$\displaystyle X_{t} = a + Z_{t} + Z_{t-1}$ a is a constant

I've got to decide if its weakly stationary or not.

$\displaystyle E[X_{t}] = E[a] + E[Z_{t}] + E[Z_{t-1}]$

which = E[a] does thie mean its time independent and finite?

Thanks for your help