That actually looks fine for me locally and on demo2 using Firefox on Windows.
But to be honest the current layout isn't good at all, as it won't work for various languages besides of English, as Compare to: has longer translations. For German it's Vergleichen mit:, which is always too long and the layout "breaks"

I see what's happening here. The calendar itself positioned absolutely, without any width, so it uses the parent's width (which is too small, so grows to fit the period). If we see in smaller screen (<600px) actually it looks good (see screenshot), because we changing the positions above the calendar, so under 600px the parent element will be the whole top_controls class.

Easy fix is too set a fix width to the calendar (of course be careful with the mobile view and max-width).