couple forgotten Dates exports

I realize there's some discussion going on here: https://github.com/JuliaLang/julia/commit/5999f53169f01521d8e2613adaa4129a8ac24449

But I think these need to be exported regardless, see https://github.com/Keno/Requests.jl/issues/35#issuecomment-53598633 and https://github.com/JuliaLang/julia/issues/8151