documentclass[letterpaper]{article}
usepackage{pgfplots,amsmath}
usepgfplotslibrary{dateplot}
pgfplotsset{compat=1.11}

begin{document}

begin{tikzpicture}

begin{axis}[date coordinates in=x,
             x tick label style={rotate=90,anchor=near xticklabel},
             xticklabel=day. hour:minute,
             date ZERO=2009-08-18]

%% I want to restrict the range of the plot over certain dates and times.


addplot[restrict x to domain=2009-08-18 09:00:2009-08-18 15:00]
    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 2:00, 020)
    (2009-08-19 3:00, 000)
    (2009-08-19 6:00, 035)
    };
 end{axis}

end{tikzpicture}
end{document}

restrict x to domain=2009-08-18 09:00:2009-08-18 15:00

xmin=2009-08-18 09:00, xmax=2009-08-18 15:00,