help with understanding why script dont give disired outcome

[Douglas Mackay]

here is part of the script I am working with :
on mouseUp
  global Gbox
  ask "Date of Birth? (eg. 23/9/1987)"
  put it into firstDate
  ask "Date of Assessment? (eg. 18/3/2001)"
  put it into secondDate
  put whichage (firstDate, secondDate) into dateholder

  if Gbox = 1
  then put "year" into field "nameYear"
  else
    put "years" into field "nameYear"
  end if
  put Gbox into field "year2"

  if item 2 of dateholder = 1
  then put "month" into field "nameMonth"
  else
    put "months" into field "nameMonth"
  end if
  put item 2 of dateholder into field "month"

  if item 3 of dateholder = 1
  then put "day" into field "nameDay"
  else
    put "days" into field "nameDay"
  end if
  put item 3 of dateholder into field "day"
end mouseUp


function whichage fromwhich,towhich
  global Gbox
  set the centuryCutoff to 35
  set useSystemDate to true
  if param(1) = "" then exit to top
  put param(1) into startdate
  put param(2) into enddate
  set the itemDelimiter to "/"

  put item 3 of startdate into GsYear
  put item 3 of enddate into GeYear

  set the itemDelimiter to comma
  convert startdate to dateitems
  convert enddate to dateitems
  put item 3 of enddate into bbmsg
  put (item 3 of enddate - item 3 of startdate) into diffjour
  if diffjour < 0 then
    add "1" to item 2 of startdate
    get item 2 of enddate
    if (it is in "1,3,5,7,8,10") or (it= "12") then put "31" into correction
    else if (it is in "4,6,9,11") then put "30" into correction
    else if item 1 of enddate mod 4 = 0 then put "29" into correction
    else put "28" into correction
    add correction to item 3 of enddate
    put (item 3 of enddate - item 3 of startdate) into diffjour
  end if
  put (item 2 of enddate - item 2 of startdate) into diffmois
  if diffmois < 0 then
    add "1" to item 1 of startdate
    put (item 2 of enddate) + 12 into item 2 of enddate
    put (item 2 of enddate - item 2 of startdate) into diffmois
    add "1" to GsYear

the line in bold type will not work when giving start and enddates 1961 , and for example todays date .having looked in the debugger  convert startdate to dateitems

 and                  convert enddate to dateitems  
only converts the enddate to dateitems 

can anyone explain why this happens please to me ?

Douglas