jQuery.param({foo: 1}); // => "foo=1" - SUCCESS! jQuery.param({bar: new Date()}); // => "" - OUCH! There is no problem with encodeURIComponent(new Date()), which is what I would have thought param is calling for each member. Also, explicitly using "traditional" param (e.g. jQuery.param(xxx, true)) DOES serialize the date, but alas, that isn't of much help since my data structure isn't flat. Is this because typeof(Date) == "object" and param tries to descend into it to find scalar values? How might one realistically serialize an object that happens to have Date's in it for $.post() etc.?