documentclass{article}

usepackage{pgfplots}
pgfplotsset{compat=newest}

pagestyle{empty}
usepgfplotslibrary{dateplot}

begin{document}
% requires usepgfplotslibrary{dateplot} !
begin{tikzpicture}
  begin{axis}[
    date coordinates in=x,
    xtick=data,
    xticklabel style=
    {rotate=90,anchor=near xticklabel},
    xticklabel=day. hour:minute,
    date ZERO=2009-08-18
  ]
  addplot coordinates {
    (2009-08-18 09:00,  050)
    (2009-08-18 12:00,  100)
    (2009-08-18 15:00,  100)
    (2009-08-18 18:35,  100)
    (2009-08-18 21:30,  040)
    (2009-08-19,        020)
    (2009-08-19 3:00,   000)
    (2009-08-19 6:0,    035)
  };
  end{axis}
end{tikzpicture}

end{document}

documentclass{article}

usepackage{pgfplots}
pgfplotsset{compat=newest}

pagestyle{empty}
usepgfplotslibrary{dateplot}

makeatletter
newcommand{nonmiltime}{%
  ifnumhour>12
    expandaftertwo@digitsexpandafter{thenumexprhour-12relax}%
  else
    expandaftertwo@digitsexpandafter{hour}%
  fi
  :minute,%
  ifnumhour>11
    textsc{pm}%
  else
    textsc{am}%
  fi
}
makeatother

begin{document}
% requires usepgfplotslibrary{dateplot} !
begin{tikzpicture}
  begin{axis}[
    date coordinates in=x,
    xtick=data,
    xticklabel style=
        {rotate=90,anchor=near xticklabel},
        xticklabel=day. nonmiltime,
        date ZERO=2009-08-18
  ]
  addplot coordinates {
    (2009-08-18 09:00,  050)
    (2009-08-18 12:00,  100)
    (2009-08-18 15:00,  100)
    (2009-08-18 18:35,  100)
    (2009-08-18 21:30,  040)
    (2009-08-19,        020)
    (2009-08-19 3:00,   000)
    (2009-08-19 6:0,    035)
  };
  end{axis}
end{tikzpicture}

end{document}