documentclass{article}
usepackage{spreadtab}
begin{document}
  begin{spreadtab}{{tabular}{llll|l}}
  @ a    & @ xyz      & @ duration      & @ starttime & @ endtime \
  @ a1   & 5          & 00:03           & 00:00       & add(b2, d2) \
  @ a2   & 9          & 00:01           & (automatically value of previous endtime)       & add(b3, d3) \
  // and so on
  end{spreadtab}
end{document}