documentclass{minimal}
usepackage{pgfplots}
usepgfplotslibrary{dateplot}
begin{filecontents}{data.dat}
date            value1
2010-01-01      2
2010-01-02      10
2010-01-03      8
2010-01-04      15
end{filecontents}
usepackage{pgfplotstable}
begin{document}
begin{tikzpicture}

begin{axis}[
date coordinates in=x,
xticklabel={day.month.year},
xmin={2010-01-01},
xmax={2010-01-05},x tick label style={yshift=-3pt,xshift=-4pt,rotate=-30,anchor=west}]
addplot table [x=date,y=value1] {data.dat};
addplot table [x=date, y={create col/linear regression={y=value1}}] {data.dat};
end{axis}

end{tikzpicture}
end{document}

documentclass{minimal}
usepackage{pgfplots}
usepgfplotslibrary{dateplot}
begin{filecontents}{data.dat}
date            value1
2010-01-01      2
2010-01-02      10
2010-01-03      8
2010-01-04      15
end{filecontents}
usepackage{pgfplotstable}

% formats row ticknum of the input table using '#3'
% #1: table name
% #2: column name
% #3: formatting instruction like day.month.year
newcommanddatelabelsfromtable[3]{%
    begingroup
    countdefresult=count200 %
    pgfplotstablegetelem{ticknum}{#2}of{#1}%
    pgfcalendardatetojulian{pgfplotsretval}{result}%
    pgfcalendarjuliantodate{result}yearmonthday
    %
    #3%
    endgroup
}

begin{document}
begin{tikzpicture}

begin{axis}[
    x tick label style={yshift=-3pt,xshift=-4pt,rotate=-30,anchor=west},
    xticklabel={datelabelsfromtable{data.dat}{date}{day.month.year}},
    xtick=data,
]
addplot table [x expr=coordindex,y=value1] {data.dat};
addplot table [x expr=coordindex, y={create col/linear regression={y=value1}}] {data.dat};
end{axis}

end{tikzpicture}
end{document}

documentclass{minimal}
usepackage{pgfplots}
usepgfplotslibrary{dateplot}
begin{filecontents}{data.dat}
date            value1
2010-01-01      2
2010-01-02      10
2010-01-03      8
2010-01-04      15
end{filecontents}
usepackage{pgfplotstable}
begin{document}
begin{tikzpicture}

begin{axis}[
    x tick label style={yshift=-3pt,xshift=-4pt,rotate=-30,anchor=west},
    xticklabels={01.01.2010, 02.01.2010, 03.01.2010, 04.01.2010},
    xtick=data,
]
addplot table [x expr=coordindex,y=value1] {data.dat};
addplot table [x expr=coordindex, y={create col/linear regression={y=value1}}] {data.dat};
end{axis}

end{tikzpicture}
end{document}