In my opinion the safest way is to define your own title page and play around with the spacing as you like.
\documentclass{beamer}
\title{Some title in the middle of the page}
\date{Some date in the bottom of the page}
\makeatletter
\setbeamertemplate{title page}{%
\vbox{}
% \vfill
\vspace{3.5cm}% NEW
\begingroup
\centering
\begin{beamercolorbox}[sep=8pt,center]{title}
\usebeamerfont{title}\inserttitle\par%
\ifx\insertsubtitle\@empty%
\else%
\vskip0.25em%
{\usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\insertsubtitle\par}%
\fi%
\end{beamercolorbox}%
\vskip1em\par
\begin{beamercolorbox}[sep=8pt,center]{author}
\usebeamerfont{author}\insertauthor
\end{beamercolorbox}
\begin{beamercolorbox}[sep=8pt,center]{institute}
\usebeamerfont{institute}\insertinstitute
\end{beamercolorbox}
\vspace{1.5cm}% NEW
\begin{beamercolorbox}[sep=8pt,center]{date}
\usebeamerfont{date}\insertdate
\end{beamercolorbox}\vskip0.5em
% {\usebeamercolor[fg]{titlegraphic}\inserttitlegraphic\par}
\endgroup
% \vfill
}
\makeatother
\begin{document}
\frame{\titlepage}
\end{document}