input = {{"date_start"}, {19900201}, {19900201}, {19980201}, {20010201}};
expectedResult = {"date_start", 1990, 1990, 1998, 2001};
One very literal approach would be:
result = MapAt[Quotient[#, 10000] &, Flatten@input, 2 ;;]
But you mentioned "dateobject". Are you actually wanting DateObject
s?
MapAt[DateObject[{Quotient[#, 10000], Quotient[Mod[#, 10000], 100], Mod[#, 100]}] &, Flatten@input, 2 ;;]