% first, define a variable to hold the formatted date:
date = #(strftime "%d-%m-%Y" (localtime (current-time)))

% use it in the title block:
\header {
  title = "Including the date!"
  subtitle = \date
}

\score {
  \relative c'' {
    c4 c c c
  }
}
% and use it in a \markup block:
\markup {
  \date
}