Two neutrino double decay $2 uetaeta$

A feynman diagram for two neutrino double beta decay \(2\nu\beta\beta\). Time obviously points upwards rather than sideways from left to right. As my professor said, "I don't understand why people use left to right to indicate time. Afterall Feynman himself used bottom to top."

\begin{tikzpicture}
  \begin{feynman}
    \vertex (a);
    \vertex [below left=2cm and 0.4cm of a] (b);
    \vertex [above left=2cm and 0.4cm of a] (c);
    \vertex [above right=of a] (d);
    \vertex [above left=1cm and 0.4cm of d] (e);
    \vertex [above right=1cm and 0.4cm of d] (f);
    \diagram*{
        (b)--[fermion,edge label={$n^0$}] (a) --[fermion,edge label={$p^{+}$}] (c),
        (a) --[boson,edge label'={\small $W^{-}$}] (d),
        (d) --[fermion,edge label={$e^{-}$}] (e),
        (f) --[fermion,edge label'={\tiny $\bar{\nu}$}] (d)
    };
    \vertex [right=3.5cm of a] (aa);
    \vertex [below right=2cm and 0.4cm of aa] (ab);
    \vertex [above right=2cm and 0.4cm of aa] (ac);
    \vertex [above left=of aa] (ad);
    \vertex [above left=1cm and 0.4cm of ad] (ae);
    \vertex [above right=1cm and 0.4cm of ad] (af);
    \diagram*{
        (ab)--[fermion,edge label'={$n^0$}] (aa) --[fermion, edge label'={$p^{+}$}] (ac),
        (aa) --[boson,edge label={\small $W^{-}$}] (ad),
        (ae) --[fermion,edge label={\tiny $\bar{\nu}$}] (ad),
        (ad) --[fermion,edge label'={ $e^{-}$}] (af)
    };
  \end{feynman}
\end{tikzpicture}