DateTimeFormat.format_part
method _type : Js_of_ocaml__.Js.js_string Js_of_ocaml__.Js.t Js_of_ocaml__.Js.readonly_prop
method _value : Js_of_ocaml__.Js.js_string Js_of_ocaml__.Js.t Js_of_ocaml__.Js.readonly_prop