How can I calculate the cross variation between a standard Brownian motion $(B_t)_{t\geq 0}$ and the process $(B^T_{t})_{t\geq T}$ given by $B^T_t= B_t-B_{t-T}$? Here $T$ is just a positive number.

I tried to use the definition of cross variation. So, let $t\geq T$ and $\Pi=\{t_0=T, t_1, \cdots, t_n=t\}$ be a partition of the interval $[T,t]$. Then

$$\langle B, {B}^{T}\rangle_t = \lim_{\|\Pi\| \rightarrow 0} \sum_{i=1}^n (B_{t_{i}}- B_{t_{i-1}})({B}_{t_{i}}^{T}- {B}^{T}_{t_{i-1}})$$ where $\|\Pi\|=\max_{1\leq i\leq n}(t_i-t_{i-1})$.

Then $$\langle B, {B}^{T}\rangle_t = \lim_{\|\Pi\| \rightarrow 0} \sum_{i=1}^n (B_{t_{i}} -B_{t_{i-1}})^2 - \lim_{\|\Pi\| \rightarrow 0} \sum_{i=1}^n (B_{t_{i}} -B_{t_{i-1}}) (B_{t_{i}-T}- B_{t_{i-1}-T})\\ = (t-T) - \lim_{\|\Pi\| \rightarrow 0} \sum_{i=1}^n (B_{t_{i}} -B_{t_{i-1}}) (B_{t_{i}-T}- B_{t_{i-1}-T})$$ where the last equation follows since the quadratic variation of the standard Brownian motion on $[T, t]$ is just $t-T$.