Use time format from the standard library

Ideally I would like to use `std::put_time()` with `std::ios::imbue()` to print times locally formatted but GCC does not support `std::put_time()` yet. I have a working prototype using `std::setlocale()` but that changes the locale globally and I'm not sure that's a good thing; `std::ios::imbue()` would just change the formatting for that stream.