The plots can't be directly combined because the PlotRange of the DateListPlot is far out to the right on the horizontal axis, since it corresponds to a large AbsoluteTime value. So we have to convert the ErrorListPlot to the same coordinate range before combining the plots using Show:
Needs["ErrorBarPlots`"]
data = {{{2006, 10, 1}, 10, 1}, {{2006, 10, 15},
12, .1}, {{2006, 10, 30}, 15, 2}, {{2006, 11, 20}, 20, 1}};
data2 =
Map[{{AbsoluteTime[#[[1]]], #[[2]]}, ErrorBar[#[[3]]]} &, data]
(*
==> {{{3368649600, 10}, ErrorBar[1]}, {{3369859200, 12},
ErrorBar[0.1]}, {{3371155200, 15}, ErrorBar[2]}, {{3372969600, 20},
ErrorBar[1]}}
*)
errorp = ErrorListPlot[data2];
datep = DateListPlot[data[[All, 1 ;; 2]]];
Show[datep, errorp]
