data1 = RandomChoice[DateRange[{2010, 1, 1}, {2010, 5, 31}, "Day"],
2000];
data2 = RandomChoice[DateRange[{2010, 3, 1}, {2010, 8, 31}, "Day"],
2000];
CreateDocument[
DateHistogram[{data1, data2}],
StyleDefinitions -> #,
WindowSize -> All
] & /@ {"Dialog.nb", "Default.nb"}