Btukfyl

why the line of 2008 above the box of 2007? I did not change any code. I want to change the line of 2008 below of the box of 2007.
Chronosys printing measurement units

documentclass[border=10pt,multi,tikz]{standalone}

%usepackage[UTF8]{ctex}

usepackage{pgfcalendar}

usepackage{datenumber,xparse,fp}

usepackage{enumitem}

%usepackage{lipsum}

usetikzlibrary{arrows.meta,backgrounds,fixedpointarithmetic}

makeatletter

ExplSyntaxOn

tl_new:N l_chronos_date_tl

tl_new:N l_chronos_dateformat_tl

tl_new:N l_chronos_year_tl

tl_new:N l_chronos_yearformat_tl

tl_set:Nn l_chronos_dateformat_tl { !d/!m/!Y }

tl_set:Nn l_chronos_yearformat_tl { !Y }

% YY yn lle YYYY

cs_new_protected_nopar:Npn chronos_year_shorten:n #1

{

  chronos_year_shorten_aux:w #1 q_stop % expl3 manuaal, 46

}

cs_new_protected_nopar:Npn chronos_year_shorten_aux:w #1 #2 #3 #4 q_stop

{

  #3 #4

}

cs_generate_variant:Nn chronos_year_shorten:n { V , c }

cs_generate_variant:Nn int_abs:n { c }

cs_generate_variant:Nn tl_replace_all:Nnn { Nnx }

% dangos dyddiadau

cs_new_protected_nopar:Npn chronos_show_date:n #1

{% ateb Joseph Wright: http://tex.stackexchange.com/a/327642/

  tl_set_eq:NN l_chronos_date_tl l_chronos_dateformat_tl

  tl_replace_all:Nnx l_chronos_date_tl { !a } { pgfcalendarweekdayshortname{thechronos@weekday} }

  tl_replace_all:Nnx l_chronos_date_tl { !A } { pgfcalendarweekdayname{thechronos@weekday} }

  tl_replace_all:Nnx l_chronos_date_tl { !b } { pgfcalendarmonthshortname{csname chronos@#1monthendcsname} }

  tl_replace_all:Nnx l_chronos_date_tl { !B } { pgfcalendarmonthname{csname chronos@#1monthendcsname} }

  tl_replace_all:Nnx l_chronos_date_tl { !d } { csname chronos@#1dayendcsname }

  tl_replace_all:Nnx l_chronos_date_tl { !E } { chronos_dateformat_era:c { chronos@#1year } }

  tl_replace_all:Nnx l_chronos_date_tl { !m } { csname chronos@#1monthendcsname }

  tl_replace_all:Nnx l_chronos_date_tl { !q } { chronos_dateformat_sign:c { chronos@#1year } }

  tl_replace_all:Nnx l_chronos_date_tl { !Q } { chronos_dateformat_signs:c { chronos@#1year } }

  tl_replace_all:Nnx l_chronos_date_tl { !y } { chronos_year_shorten:c { chronos@#1year } }

  tl_replace_all:Nnx l_chronos_date_tl { !Y } { int_abs:c { chronos@#1year } }

  l_chronos_date_tl

}

cs_new_protected_nopar:Npn chronos_show_year:n #1

{% ateb Joseph Wright: http://tex.stackexchange.com/a/327642/

  tl_set_eq:NN l_chronos_year_tl l_chronos_yearformat_tl

  tl_replace_all:Nnx l_chronos_year_tl { !E } { chronos_dateformat_era:n { #1 } }

  tl_replace_all:Nnx l_chronos_year_tl { !q } { chronos_dateformat_sign:n { #1 } }

  tl_replace_all:Nnx l_chronos_year_tl { !Q } { chronos_dateformat_signs:n { #1 } }

  tl_replace_all:Nnx l_chronos_year_tl { !y } { chronos_year_shorten:n { #1 } }

  tl_replace_all:Nnx l_chronos_year_tl { !Y } { int_abs:n { #1 } }

  l_chronos_year_tl

}

cs_new_protected_nopar:Npn chronos_dateformat_sign:n #1

{

  int_compare:nT { #1 < 0 } { - }

}

cs_generate_variant:Nn chronos_dateformat_sign:n { c }

cs_new_protected_nopar:Npn chronos_dateformat_signs:n #1

{

  int_compare:nTF

  { #1 < 0 } { - }

  {

    int_compare:nT { #1 > 0 }

    {

      ifchronos@yearzerorelax

      else

        int_compare:nT { #1 > 1} { +  }

      fi

    }

  }

}

cs_generate_variant:Nn chronos_dateformat_signs:n { c }

cs_new_protected_nopar:Npn chronos_dateformat_era:n #1

{

  int_compare:nTF

  { #1 < 0 } { chronos@yearbce }

  {

    int_compare:nT { #1 > 0 }

    {

      ifchronos@yearzerorelax

      else

        int_compare:nT { #1 > 1} { chronos@yearce }

      fi

    }

  }

}

cs_generate_variant:Nn chronos_dateformat_era:n { c }

cs_new_protected_nopar:Npn chronos_set_dateformat:n #1

{

  tl_set:Nn l_chronos_dateformat_tl { #1 }

  tl_replace_all:Nnn l_chronos_dateformat_tl { ~ } { c_space_token }

}

cs_new_protected_nopar:Npn chronos_set_yearformat:n #1

{

  tl_set:Nn l_chronos_yearformat_tl { #1 }

  tl_replace_all:Nnn l_chronos_yearformat_tl { ~ } { c_space_token }

}

% user interface

NewDocumentCommand chronos@setdateformat { m }

{

  chronos_set_dateformat:n { #1 }

}

NewDocumentCommand chronos@setyearformat { m }

{

  chronos_set_yearformat:n { #1 }

}

% for pgf/tikz convenience

NewDocumentCommand chronos@showdate { o m }

{

  group_begin:

    IfValueT { #1 }

    {

      chronos_set_dateformat:n { #1 }

    }

    pgfcalendarjuliantoweekday{csname thechronos@#2dateendcsname}{c@chronos@weekday}%

    chronos_show_date:n { #2 }

  group_end:

}

NewDocumentCommand chronos@showyear { o m }

{

  group_begin:

    IfValueT { #1 }

    {

      chronos_set_yearformat:n { #1 }

    }

    chronos_show_year:n { #2 }

  group_end:

}

ExplSyntaxOff

newlengthchronos@tempdima

newcounter{chronos@date}

newcounter{chronos@startdate}

newcounter{chronos@enddate}

newcounter{chronos@startyear}

newcounter{chronos@endyear}

newcounter{chronos@yeardate}

newcounter{chronos@thingdate}

newcounter{chronos@otherthingdate}

newcounter{chronos@weekday}

newcounter{chronos@tempcnta}

newififchronos@marks

chronos@marksfalse

newififchronos@timeline@showyears

chronos@timeline@showyearstrue

newififchronos@eventyearsonline

chronos@eventyearsonlinefalse

newififchronos@yearzero

chronos@yearzerofalse

newififchronos@onlytext

chronos@onlytextfalse

newififchronos@markeras

chronos@markerasfalse

newififchronos@yearsonline

chronos@yearsonlinefalse

newififchronos@eventdatessplit

chronos@eventdatessplitfalse

pgfkeys{/pgf/number format,

  int detect,

  set thousands separator={},

}

tikzset{%

  /handlers/.chronos too/.code={%

    edefchronos@tempc{pgfkeyscurrentpath}%

    edefchronos@tempd{#1}%

    tikzset{%

      pgfkeyscurrentpath @too/.code={%

        tikzset{%

          /chronos/chronos@tempd/.append style={##1},

        }%

      },

      chronos@tempc/.forward to=chronos@tempc @too,

    }%

  },

  chronos/.code={% http://tex.stackexchange.com/a/159856/ - Claudio Fiandrino

    tikzset{%

      align=center,

      anchor=mid,

      fixed point arithmetic,

      /chronos/.cd,

      #1,

      timeline config,

      timeline config/.code={},

    }%

    ifnumchronos@startyear=chronos@endyear

      tikzset{%

        /chronos/timeline no years,

      }%

    fi

    setlengthchronos@tempdima{chronos@timelinemargin}%

    ifchronos@markeras

      {%

        chronos@timelinefont

        pgfmathsetmacrochronos@tempe{((width("chronos@bce"))>(width("chronos@ce"))) ? (width("chronos@bce")) : (width("chronos@ce"))}%

        xdefchronos@tempf{chronos@tempe}%

      }%

      addtolengthchronos@tempdima{chronos@tempf pt}%

    fi

    pgfmathsetmacrochronos@unit{(chronos@width-2*chronos@tempdima)/(thechronos@enddate-thechronos@startdate)}%

    draw [/chronos/timeline@line, line width=chronos@height] (-chronos@tempdima,0) coordinate (chronos pre) -- +(chronos@width,0) coordinate (chronos post);

    coordinate (chronos base) at (0,-.5*chronos@height);

    coordinate (chronos top) at (0,.5*chronos@height);

    coordinate (chronos foot) at (0,{-.5*chronos@height-chronos@borderheight});

    coordinate (chronos head) at (0,{.5*chronos@height+chronos@borderheight});

    coordinate (chronos start) at (0,0);

    coordinate (chronos end) at ([xshift=-chronos@tempdima]chronos post);

    ifdimchronos@borderheight>0pt

      fill [bottom color=chronos@borderinner, top color=chronos@borderouter] (chronos pre |- chronos head) rectangle (chronos post |- chronos top);

      fill [top color=chronos@borderinner, bottom color=chronos@borderouter] (chronos pre |- chronos base) rectangle (chronos post |- chronos foot);

    fi

    ifchronos@timeline@showyears

      pgfmathsetcounter{chronos@startyear}{chronos@startyear}%

      pgfmathsetcounter{chronos@endyear}{chronos@endyear}%

      deftempa{none}%

      ifxchronos@stepfromtempa

        deftempa{01}%

        ifxchronos@startmonthtempa

          ifxchronos@startdaytempa

            elsestepcounter{chronos@startyear}%

          fi

          elsestepcounter{chronos@startyear}%

        fi

      else

        pgfmathsetcounter{chronos@startyear}{chronos@stepfrom}%

      fi

      deftempa{12}%

      deftempb{31}%

      ifxchronos@endmonthtempa

        ifxchronos@enddaytempb

          stepcounter{chronos@endyear}%

        fi

      fi

      ifnumvalue{chronos@endyear}<value{chronos@startyear}setcounter{chronos@endyear}{thechronos@startyear}fi

      pgfmathsetmacrochronos@nextstep{int(((thechronos@startyear+chronos@stepyears)>thechronos@endyear) ? thechronos@endyear : (thechronos@startyear+chronos@stepyears))}%

      ifchronos@yearzerosetcounter{chronos@tempcnta}{1}elsesetcounter{chronos@tempcnta}{0}fi

      foreach b [evaluate=b as i using {((b==0)&&(thechronos@tempcnta==0)) ? 1 : int(b)}, remember=i as ilast (initially pi)] in {thechronos@startyear,chronos@nextstep,...,thechronos@endyear} {%

        ifnumi=ilastrelax

        else

          pgfcalendardatetojulian{{i}-01-01}{c@chronos@yeardate}%

          ifchronos@yearzerorelaxelseifnum0<iaddtocounter{chronos@yeardate}{-366}fifi

          pgfmathsetmacrochronos@tempa{(thechronos@yeardate-thechronos@startdate)*chronos@unit}%

          ifchronos@yearsonline

            node (chronos@year@i) [/chronos/.cd, timeline@years, timeline year on line] at (chronos@tempa pt,0) {chronos@timelinefontchronos@showyear{i}};

            ifchronos@marks

               path [/chronos/timeline mark on line] (chronos@year@i.south) -- (chronos@year@i |- chronos base);

               path [/chronos/timeline mark on line] (chronos@year@i.north) --  (chronos@year@i |- chronos top);

            fi

            ifchronos@markeras

              path (chronos pre) +(chronos@timelinemargin,0) node (chronos@bce) [/chronos/.cd, timeline@years, timeline year on line] {chronos@timelinefontchronos@bce};

              path (chronos post) +(-chronos@timelinemargin,0) node (chronos@ce) [/chronos/.cd, timeline@years, timeline year on line] {chronos@timelinefontchronos@ce};

            fi

          else

            node (chronos@year@i) [/chronos/.cd, timeline@years, timeline year off line] at (chronos@tempa pt,0) {chronos@timelinefontchronos@showyear{i}};

            ifchronos@marks

              path [shorten <=.5*chronos@height, /chronos/timeline mark off line] (chronos@tempa pt, 0) -- (chronos@year@i);

            fi

            ifchronos@markeras

              path (chronos pre) +(chronos@timelinemargin,0) node (chronos@bce) [/chronos/.cd, timeline@years, timeline year off line] {chronos@timelinefontchronos@bce};

              path (chronos post) +(-chronos@timelinemargin,0) node (chronos@ce) [/chronos/.cd, timeline@years, timeline year off line] {chronos@timelinefontchronos@ce};

            fi

          fi

        fi

      }%

    else

      ifchronos@markeras

        path (chronos pre) +(chronos@timelinemargin,0) node (chronos@bce) [/chronos/.cd, timeline@years, timeline year off line] {chronos@timelinefontchronos@bce};

        path (chronos post) +(-chronos@timelinemargin,0) node (chronos@ce) [/chronos/.cd, timeline@years, timeline year off line] {chronos@timelinefontchronos@ce};

      fi

    fi

    ifchronos@eventyearsonline

      tikzset{%

        /chronos/timeline years=on line,

      }%

    fi

  },

  chronos set date/.code args={#1:#2:#3:#4}{%

    pgfcalendardatetojulian{{#1}-#2-#3}{c@chronos@date}%

    setcounter{chronos@#4date}{thechronos@date}%

    ifchronos@yearzerorelax

    else

      setcounter{chronos@tempcnta}{#1}%

      ifnum0<value{chronos@tempcnta}%

        addtocounter{chronos@#4date}{-366}%

      fi

    fi

    expandafterdefcsname chronos@#4yearendcsname{#1}%

    expandafterdefcsname chronos@#4monthendcsname{#2}%

    expandafterdefcsname chronos@#4dayendcsname{#3}%

  },

  chronos date/.style args={#1-#2-#3}{%

    /tikz/chronos set date/.expanded={#1:#2:#3:thing}%

  },

  chronos period date/.style args={#1-#2-#3}{%

    /tikz/chronos set date/.expanded={#1:#2:#3:otherthing}%

  },

  /chronos/.search also={/tikz},

  /chronos/.cd,

  timeline config/.code={},

  date format/.code={%

    chronos@setdateformat{#1}%

  },

  year format/.code={%

    chronos@setyearformat{#1}%

  },

  step years/.store in=chronos@stepyears,

  step from year/.store in=chronos@stepfrom,

  start date/.style args={#1-#2-#3}{%

    /chronos/timeline config/.append code={%

      tikzset{/tikz/chronos set date/.expanded={#1:#2:#3:start}}%

    },

  },

  end date/.style args={#1-#2-#3}{%

    /chronos/timeline config/.append code={%

      tikzset{/tikz/chronos set date/.expanded={#1:#2:#3:end}}%

    },

  },

  ce year label/.store in=chronos@yearce,

  bce year label/.store in=chronos@yearbce,

  timeline ce label/.store in=chronos@ce,

  timeline bce label/.store in=chronos@bce,

  timeline width/.store in=chronos@width,

  timeline height/.store in=chronos@height,

  width/.forward to=/chronos/timeline width,

  height/.forward to=/chronos/timeline height,

  timeline border height/.store in=chronos@borderheight,

  timeline border inner colour/.code={colorlet{chronos@borderinner}{#1}},

  timeline border outer colour/.code={colorlet{chronos@borderouter}{#1}},

  timeline mark eras/.is if=chronos@markeras,

  timeline margin/.store in=chronos@timelinemargin,

  timeline font/.store in=chronos@timelinefont,

  timeline years set/.store in=chronos@timelineyears,

  timeline years/.is choice,

  timeline years/.forward to=/chronos/timeline years set,

  timeline years/above/.code={%

    tikzset{%

      /chronos/timeline@years/.style={above, anchor=south, yshift=.5*chronos@height},

    }%

  },

  timeline years/below/.code={%

    tikzset{%

      /chronos/timeline@years/.style={below, anchor=north, yshift=-.5*chronos@height},

    }%

  },

  timeline years/on line/.code={%

    chronos@yearsonlinetrue

    tikzset{%

      /chronos/timeline@years/.style={anchor=center},

    }%

  },

  only years/.code={%

    edeftempa{}%

    edeftempb{#1}%

    ifxtempatempbchronos@setdateformat{!Y}elsechronos@setdateformat{#1}fi

    tikzset{%

      /tikz/chronos date/.style={%

        /tikz/chronos set date/.expanded={##1:01:01:thing}%

      },

      /tikz/chronos period date/.style={%

        /tikz/chronos set date/.expanded={##1:01:01:otherthing}%

      },

    }%

  },

  only text/.code={%

    tikzset{/chronos/only years}%

    chronos@setdateformat{}%

    chronos@onlytexttrue

  },

  year zero/.is if=chronos@yearzero,

  background/.code={%

    colorlet{chronos@background}{#1}%

  },

  timeline marks/.is if=chronos@marks,

  timeline show years/.is if=chronos@timeline@showyears,

  timeline no years/.code={%

    tikzset{%

      /chronos/timeline show years=false,

    }%

  },

  lines/.style={draw, {Triangle[width=0pt 3,reversed,length=0pt 1.5]}-{Triangle[width=0pt 3,reversed,length=0pt 1.5]}, shorten <={.5*chronos@height+chronos@borderheight}},

  events/.style={fill=chronos@background, fill opacity=.75, text opacity=1, draw opacity=1, rounded corners, align=center, font=footnotesize},

  period/.style={draw},

  period event line/.style={/chronos/lines},

  period event/.style={/chronos/events},

  event line/.style={/chronos/lines},

  event/.style={/chronos/events},

  event years on line/.is if=chronos@eventyearsonline,

  event year on line/.style={/chronos/timeline@years, /chronos/timeline year on line},

  event dates split/.is if=chronos@eventdatessplit,

  event date split/.style={/chronos/event},

  event distance/.store in=chronos@eventdistance,

  special date/.store in=chronos@specialdate,

  timeline@line/.style={draw},

  timeline line/.chronos too=timeline@line,

  timeline year off line/.style={fill=chronos@background, text opacity=1, align=center, fill opacity=.75},

  timeline mark off line/.style={draw, {Triangle[width=0pt 3,reversed,length=0pt 1.5]}-, thin, shorten >=-2pt},

  timeline year on line/.style={text=chronos@background, inner sep=1pt, align=center},

  timeline mark on line/.style={draw=chronos@background, shorten >=1.5pt},

  timeline mark too/.code={%

    tikzset{%

      /chronos/.cd,

      timeline mark on line/.append style={#1},

      timeline mark off line/.append style={#1},

    }%

  },

  timeline year too/.code={%

    tikzset{%

      /chronos/.cd,

      timeline year on line/.append style={#1},

      timeline year off line/.append style={#1},

    }%

  },

  timeline mark/.forward to=/chronos/timeline mark too,

  timeline year/.forward to=/chronos/timeline year too,

  start date=1001-10-01,

  end date=1003-06-14,

  timeline width=100mm,

  timeline height=1pt,

  timeline border height=0pt,

  chronos date=1850-01-01,

  chronos period date=1851-01-01,

  step years=1,

  timeline years=above,

  background=white,

  timeline border inner colour=black,

  timeline border outer colour=chronos@background,

  step from year=none,

  special date=none,

  ce year label={textsc{ce}},

  bce year label={textsc{bce}},

  event distance=-10pt,

  timeline ce label={CE},

  timeline bce label={BCE},

  timeline margin=10pt,

  timeline font=sffamily,

}

NewDocumentCommand chronosevent { O {} m O {} +m D () { chronos@eventdistance } }

{%

  tikzset{%

    chronos date/.expanded={#2},

  }%

  pgfmathsetmacrochronos@offset{(#5 < 0pt) ? (#5-.5*chronos@height-chronos@borderheight) : (#5+.5*chronos@height+chronos@borderheight)}%

  pgfmathsetmacrochronos@anchor{(#5 < 0pt) ? "north" : "south"}%

  scoped[on background layer]{%

    ifchronos@eventdatessplit

      ifchronos@onlytextrelax

      else

        pgfmathsetmacrochronos@invanchor{(#5 < 0pt) ? "south" : "north"}%

        path [postaction={/chronos/event line, #1}] ({(thechronos@thingdate-thechronos@startdate)*chronos@unit pt},0) -- +(0,-chronos@offset pt) node [anchor=chronos@invanchor, /chronos/event date split, #3] {chronos@showdate{thing}};

      fi

      path [postaction={/chronos/event line, #1}] ({(thechronos@thingdate-thechronos@startdate)*chronos@unit pt},0) -- +(0,chronos@offset pt) node [anchor=chronos@anchor, /chronos/event, #3] {#4};

    else

      path [postaction={/chronos/event line, #1}] ({(thechronos@thingdate-thechronos@startdate)*chronos@unit pt},0) -- +(0,chronos@offset pt) node [anchor=chronos@anchor, /chronos/event, #3] {ifchronos@onlytextrelaxelsechronos@showdate{thing}\fi#4};

    fi

  }%

  ifchronos@eventyearsonline

    edeftempa{none}%

    edeftempb{chronos@specialdate}%

    ifxtempatempbelseletchronos@thingyearchronos@specialdatetikzset{/chronos/special date=none}fi

    node [/chronos/.cd, event year on line] at ({(thechronos@thingdate-thechronos@startdate)*chronos@unit pt},0) {chronos@thingyear};

  fi

}

NewDocumentCommand chronosspecialdate { m }

{%

  tikzset{%

    /chronos/special date=#1,

  }%

}

NewDocumentCommand chronosperiod { O {} m O {} m O {} +m D () { chronos@eventdistance } }

{%

  tikzset{%

    chronos date/.expanded={#2}, chronos period date/.expanded={#4}

  }%

  pgfmathsetmacrochronos@offset{(#7 < 0pt) ? (#7-.5*chronos@height-chronos@borderheight) : (#7+.5*chronos@height+chronos@borderheight)}%

  pgfmathsetmacrochronos@anchor{(#7 < 0pt) ? "north" : "south"}%

  ifchronos@yearsonline

    pgfmathsetmacrochronos@borderoffset{(#7 < 0pt) ? (-.5*chronos@height-.5*chronos@borderheight) : (.5*chronos@height+.5*chronos@borderheight)}%

    path [postaction={line width=chronos@borderheight, /chronos/period, blend mode=overlay, #1}] ({(thechronos@thingdate-thechronos@startdate)*chronos@unit pt},chronos@borderoffset pt) -- ({(thechronos@otherthingdate-thechronos@startdate)*chronos@unit pt},chronos@borderoffset pt);

  else

    path [postaction={line width=chronos@height, /chronos/period, #1}] ({(thechronos@thingdate-thechronos@startdate)*chronos@unit pt},0) -- ({(thechronos@otherthingdate-thechronos@startdate)*chronos@unit pt},0);

  fi

  scoped[on background layer]{path [postaction={/chronos/period event line, #3}] ({(.5*thechronos@otherthingdate+.5*thechronos@thingdate-thechronos@startdate)*chronos@unit pt},0) -- +(0,chronos@offset pt) node [anchor=chronos@anchor, /chronos/period event, #5] {ifchronos@onlytextrelaxelsechronos@showdate{thing}--chronos@showdate{otherthing}\fi#6};}

}

makeatother

begin{document}

begin{tikzpicture}

  [

    chronos={%

      timeline width=100mm,

      start date=2007-01-01,

      end date=2019-01-01,

      events/.append style={text width=20mm},

      timeline font=footnotesize,

      timeline marks,

      timeline height=5mm,

      only text,

      timeline years=on line,

      timeline border height=5pt,

    },

    my box/.style={draw=#1, rounded corners, text=#1, thick},

  ]

  chronosevent[blue]{2007}[my box=blue]{Debate on CEPA Begins}(-5pt)

  chronosevent[green!75!black]{2008}[my box=green!75!black]{CEPA Amendments Passed}(-40pt)

  chronosevent[magenta]{2009}[text width=30mm, my box=magenta]{begin{itemize}[before=color{magenta}sffamily]

 item try

 item try

end{itemize}

}(-80pt)

  chronosevent[orange]{2011}[my box=orange]{Categorization of DSL}(-5pt)

  path [every node/.style={font=largebfseriessffamily}] (chronos start |- current bounding box.north) node (n) [above] {2007} (chronos end |- n) node {2019};

end{tikzpicture}

end{document}

Tikz shapes, like nodes etc, are usually created in the order that these commands appear in your code. So, If you give first a command:

fill[red] (0,0) rectangle (1,1);

And after that a:

fill[green] (0,0) rectangle (2,2);

The green rectangle will cover the red one and the first will be printed in your output below the green and thus it will be invisible.

Full code and output:

documentclass[border=10pt,tikz]{standalone}

begin{document}

begin{tikzpicture}

fill[red] (0,0) rectangle (1,1);

fill[green] (0,0) rectangle (2,2);

end{tikzpicture}

end{document} 

If you change the order of the above commands you will get the green below the red and so, the part of the green that is not below of the red will be visible and the red will printed over the green and will be visible and whole.

Full code and output:

documentclass[border=10pt,tikz]{standalone}

begin{document}

begin{tikzpicture}

fill[green] (0,0) rectangle (2,2);

fill[red] (0,0) rectangle (1,1);

end{tikzpicture}

end{document}

Thus in your case you have to give the commands:

chronosevent[blue]{2007}[my box=blue]{Debate on CEPA Begins}(-5pt)

  chronosevent[green!75!black]{2008}[my box=green!75!black]{CEPA Amendments Passed}(-40pt)

  chronosevent[magenta]{2009}[text width=30mm, my box=magenta]{begin{itemize}[before=color{magenta}sffamily]

in the order:

chronosevent[magenta]{2009}[text width=30mm, my box=magenta]{begin{itemize}[before=color{magenta}sffamily]

chronosevent[green!75!black]{2008}[my box=green!75!black]{CEPA Amendments Passed}(-40pt)

chronosevent[blue]{2007}[my box=blue]{Debate on CEPA Begins}(-5pt)

This way the last (2007) will be printed above the middle (2008) one and the middle
above the first (2009).