main.dart

class Foo {
  String foo;
  Foo(this.foo);
  String toString() => foo;
}

void main() {
  final map = <DateTime, List<dynamic>>{
    DateTime(2010, 10, 10): [Foo("bar"), Foo("baz"), Foo("qux")],
    DateTime(2011, 10, 10): [Foo("bar"), Foo("baz"), Foo("qux")],
    DateTime(2012, 10, 10): [Foo("bar"), Foo("baz"), Foo("qux")],
  };
  
 map[DateTime.parse('2011-10-10')].removeWhere((foo) => foo.foo == "baz");
  
  print(map);
}